Correct bi_zeros and pbvv_seq docstrings; add references for all specfun functions
Correct bi_zeros and pbvv_seq docstrings; add references for all specfun functions

The bi_zeros and pbvv_seq docstrings were incorrectly copied from ai_zeros and
pbdv_seq, respectively.  This corrects those docstrings and adds some more
detail.

The `specfun` functions all come from "Computation of Special Functions" by
Zhang and Jin, so I've added a reference to that work to each function that
uses `specfun`.

This closes #4956 and closes #4957.

grlee77/scipy,surhudm/scipy,haudren/scipy,dominicelse/scipy,Gillu13/scipy,rmcgibbo/scipy,zerothi/scipy,Stefan-Endres/scipy,endolith/scipy,jor-/scipy,ilayn/scipy,mgaitan/scipy,woodscn/scipy,rmcgibbo/scipy,andyfaff/scipy,andim/scipy,fernand/scipy,pschella/scipy,Newman101/scipy,endolith/scipy,sauliusl/scipy,petebachant/scipy,vberaudi/scipy,fernand/scipy,Shaswat27/scipy,jonycgn/scipy,aeklant/scipy,nmayorov/scipy,jamestwebber/scipy,vigna/scipy,gertingold/scipy,andyfaff/scipy,andim/scipy,jakevdp/scipy,richardotis/scipy,mikebenfield/scipy,haudren/scipy,mdhaber/scipy,pbrod/scipy,gdooper/scipy,argriffing/scipy,behzadnouri/scipy,giorgiop/scipy,mikebenfield/scipy,tylerjereddy/scipy,felipebetancur/scipy,aman-iitj/scipy,cpaulik/scipy,felipebetancur/scipy,ilayn/scipy,pnedunuri/scipy,perimosocordiae/scipy,scipy/scipy,giorgiop/scipy,Stefan-Endres/scipy,tylerjereddy/scipy,larsmans/scipy,haudren/scipy,richardotis/scipy,aarchiba/scipy,nonhermitian/scipy,dominicelse/scipy,larsmans/scipy,niknow/scipy,Kamp9/scipy,jor-/scipy,kalvdans/scipy,kleskjr/scipy,Shaswat27/scipy,zerothi/scipy,futurulus/scipy,dominicelse/scipy,mtrbean/scipy,pbrod/scipy,aeklant/scipy,Stefan-Endres/scipy,piyush0609/scipy,arokem/scipy,fredrikw/scipy,gfyoung/scipy,person142/scipy,josephcslater/scipy,pbrod/scipy,jamestwebber/scipy,behzadnouri/scipy,maniteja123/scipy,WarrenWeckesser/scipy,mingwpy/scipy,Shaswat27/scipy,zaxliu/scipy,rgommers/scipy,arokem/scipy,apbard/scipy,pyramania/scipy,trankmichael/scipy,ndchorley/scipy,WillieMaddox/scipy,aeklant/scipy,pizzathief/scipy,vigna/scipy,piyush0609/scipy,Srisai85/scipy,ChanderG/scipy,ortylp/scipy,josephcslater/scipy,surhudm/scipy,argriffing/scipy,jjhelmus/scipy,jonycgn/scipy,minhlongdo/scipy,anielsen001/scipy,gfyoung/scipy,chatcannon/scipy,chatcannon/scipy,chatcannon/scipy,sauliusl/scipy,haudren/scipy,gef756/scipy,andim/scipy,befelix/scipy,matthew-brett/scipy,felipebetancur/scipy,pnedunuri/scipy,larsmans/scipy,arokem/scipy,perimosocordiae/scipy,gfyoung/scipy,hainm/scipy,perimosocordiae/scipy,rgommers/scipy,giorgiop/scipy,behzadnouri/scipy,mgaitan/scipy,aman-iitj/scipy,arokem/scipy,WarrenWeckesser/scipy,sonnyhu/scipy,lukauskas/scipy,pnedunuri/scipy,niknow/scipy,WarrenWeckesser/scipy,newemailjdm/scipy,Eric89GXL/scipy,mhogg/scipy,bkendzior/scipy,lukauskas/scipy,ilayn/scipy,jjhelmus/scipy,cpaulik/scipy,rgommers/scipy,andyfaff/scipy,pbrod/scipy,Newman101/scipy,mgaitan/scipy,zxsted/scipy,ndchorley/scipy,zxsted/scipy,gdooper/scipy,gfyoung/scipy,ortylp/scipy,endolith/scipy,Kamp9/scipy,minhlongdo/scipy,njwilson23/scipy,jseabold/scipy,hainm/scipy,Eric89GXL/scipy,rgommers/scipy,ChanderG/scipy,mgaitan/scipy,mdhaber/scipy,niknow/scipy,njwilson23/scipy,argriffing/scipy,anielsen001/scipy,maniteja123/scipy,Srisai85/scipy,jakevdp/scipy,vberaudi/scipy,aarchiba/scipy,niknow/scipy,jjhelmus/scipy,ortylp/scipy,trankmichael/scipy,felipebetancur/scipy,hainm/scipy,woodscn/scipy,kleskjr/scipy,vanpact/scipy,mtrbean/scipy,mhogg/scipy,befelix/scipy,grlee77/scipy,nonhermitian/scipy,anielsen001/scipy,rmcgibbo/scipy,scipy/scipy,newemailjdm/scipy,ales-erjavec/scipy,WillieMaddox/scipy,woodscn/scipy,jonycgn/scipy,fredrikw/scipy,person142/scipy,vanpact/scipy,jamestwebber/scipy,jakevdp/scipy,anntzer/scipy,jamestwebber/scipy,person142/scipy,endolith/scipy,gdooper/scipy,ortylp/scipy,e-q/scipy,minhlongdo/scipy,dominicelse/scipy,futurulus/scipy,vigna/scipy,ilayn/scipy,surhudm/scipy,pizzathief/scipy,fernand/scipy,apbard/scipy,niknow/scipy,pyramania/scipy,nonhermitian/scipy,ortylp/scipy,zxsted/scipy,endolith/scipy,richardotis/scipy,anielsen001/scipy,gdooper/scipy,lhilt/scipy,zaxliu/scipy,petebachant/scipy,WarrenWeckesser/scipy,perimosocordiae/scipy,vberaudi/scipy,pizzathief/scipy,zaxliu/scipy,petebachant/scipy,trankmichael/scipy,pyramania/scipy,gertingold/scipy,rmcgibbo/scipy,kleskjr/scipy,mikebenfield/scipy,mingwpy/scipy,behzadnouri/scipy,cpaulik/scipy,Kamp9/scipy,pnedunuri/scipy,ChanderG/scipy,apbard/scipy,surhudm/scipy,jakevdp/scipy,njwilson23/scipy,pschella/scipy,andim/scipy,sonnyhu/scipy,surhudm/scipy,pyramania/scipy,rmcgibbo/scipy,matthew-brett/scipy,jamestwebber/scipy,ndchorley/scipy,jjhelmus/scipy,andim/scipy,mtrbean/scipy,jseabold/scipy,gertingold/scipy,scipy/scipy,sonnyhu/scipy,vigna/scipy,kalvdans/scipy,gdooper/scipy,e-q/scipy,scipy/scipy,kalvdans/scipy,felipebetancur/scipy,cpaulik/scipy,endolith/scipy,ortylp/scipy,chatcannon/scipy,nonhermitian/scipy,mingwpy/scipy,hainm/scipy,fredrikw/scipy,pyramania/scipy,mingwpy/scipy,Newman101/scipy,matthewalbani/scipy,nmayorov/scipy,zerothi/scipy,gef756/scipy,aarchiba/scipy,newemailjdm/scipy,aeklant/scipy,jakevdp/scipy,person142/scipy,jor-/scipy,mdhaber/scipy,jseabold/scipy,dominicelse/scipy,mhogg/scipy,futurulus/scipy,andim/scipy,lhilt/scipy,pizzathief/scipy,sauliusl/scipy,jjhelmus/scipy,richardotis/scipy,andyfaff/scipy,josephcslater/scipy,ndchorley/scipy,anntzer/scipy,sriki18/scipy,zxsted/scipy,woodscn/scipy,matthew-brett/scipy,befelix/scipy,mingwpy/scipy,scipy/scipy,ilayn/scipy,zerothi/scipy,mhogg/scipy,matthew-brett/scipy,njwilson23/scipy,cpaulik/scipy,lukauskas/scipy,ilayn/scipy,pschella/scipy,futurulus/scipy,lukauskas/scipy,bkendzior/scipy,sauliusl/scipy,grlee77/scipy,grlee77/scipy,aarchiba/scipy,sonnyhu/scipy,jonycgn/scipy,giorgiop/scipy,kalvdans/scipy,Kamp9/scipy,pnedunuri/scipy,surhudm/scipy,ales-erjavec/scipy,mdhaber/scipy,larsmans/scipy,Srisai85/scipy,larsmans/scipy,matthewalbani/scipy,anielsen001/scipy,sriki18/scipy,woodscn/scipy,petebachant/scipy,gef756/scipy,WarrenWeckesser/scipy,WillieMaddox/scipy,pnedunuri/scipy,Stefan-Endres/scipy,kalvdans/scipy,grlee77/scipy,fernand/scipy,perimosocordiae/scipy,zaxliu/scipy,vberaudi/scipy,argriffing/scipy,zaxliu/scipy,jseabold/scipy,vanpact/scipy,mdhaber/scipy,lhilt/scipy,maniteja123/scipy,ChanderG/scipy,ChanderG/scipy,matthewalbani/scipy,mtrbean/scipy,aman-iitj/scipy,gertingold/scipy,trankmichael/scipy,chatcannon/scipy,kleskjr/scipy,maniteja123/scipy,Kamp9/scipy,sriki18/scipy,Shaswat27/scipy,maniteja123/scipy,bkendzior/scipy,mdhaber/scipy,petebachant/scipy,fredrikw/scipy,mhogg/scipy,gef756/scipy,mikebenfield/scipy,zerothi/scipy,sauliusl/scipy,josephcslater/scipy,Gillu13/scipy,giorgiop/scipy,e-q/scipy,richardotis/scipy,nmayorov/scipy,bkendzior/scipy,gfyoung/scipy,njwilson23/scipy,larsmans/scipy,nonhermitian/scipy,sriki18/scipy,aman-iitj/scipy,jor-/scipy,ales-erjavec/scipy,fredrikw/scipy,aeklant/scipy,Srisai85/scipy,pizzathief/scipy,piyush0609/scipy,mtrbean/scipy,haudren/scipy,newemailjdm/scipy,njwilson23/scipy,matthew-brett/scipy,sauliusl/scipy,Stefan-Endres/scipy,felipebetancur/scipy,Eric89GXL/scipy,Srisai85/scipy,lukauskas/scipy,minhlongdo/scipy,ndchorley/scipy,bkendzior/scipy,sonnyhu/scipy,piyush0609/scipy,pschella/scipy,mgaitan/scipy,tylerjereddy/scipy,gef756/scipy,Newman101/scipy,lhilt/scipy,e-q/scipy,mhogg/scipy,anntzer/scipy,gef756/scipy,gertingold/scipy,aarchiba/scipy,WillieMaddox/scipy,zxsted/scipy,tylerjereddy/scipy,lhilt/scipy,jonycgn/scipy,piyush0609/scipy,argriffing/scipy,sriki18/scipy,ales-erjavec/scipy,newemailjdm/scipy,jseabold/scipy,mtrbean/scipy,scipy/scipy,zaxliu/scipy,hainm/scipy,sonnyhu/scipy,pbrod/scipy,apbard/scipy,arokem/scipy,Shaswat27/scipy,vberaudi/scipy,befelix/scipy,matthewalbani/scipy,lukauskas/scipy,futurulus/scipy,nmayorov/scipy,Eric89GXL/scipy,Eric89GXL/scipy,Shaswat27/scipy,newemailjdm/scipy,giorgiop/scipy,mingwpy/scipy,aman-iitj/scipy,pschella/scipy,ChanderG/scipy,jseabold/scipy,maniteja123/scipy,josephcslater/scipy,hainm/scipy,rgommers/scipy,piyush0609/scipy,mgaitan/scipy,anielsen001/scipy,fredrikw/scipy,mikebenfield/scipy,jor-/scipy,WarrenWeckesser/scipy,matthewalbani/scipy,anntzer/scipy,trankmichael/scipy,zxsted/scipy,kleskjr/scipy,trankmichael/scipy,vigna/scipy,Newman101/scipy,Gillu13/scipy,haudren/scipy,ales-erjavec/scipy,apbard/scipy,richardotis/scipy,vanpact/scipy,behzadnouri/scipy,niknow/scipy,WillieMaddox/scipy,perimosocordiae/scipy,sriki18/scipy,ndchorley/scipy,Kamp9/scipy,fernand/scipy,vanpact/scipy,Gillu13/scipy,andyfaff/scipy,Srisai85/scipy,anntzer/scipy,behzadnouri/scipy,andyfaff/scipy,futurulus/scipy,aman-iitj/scipy,WillieMaddox/scipy,rmcgibbo/scipy,anntzer/scipy,Stefan-Endres/scipy,pbrod/scipy,kleskjr/scipy,jonycgn/scipy,zerothi/scipy,e-q/scipy,nmayorov/scipy,Gillu13/scipy,minhlongdo/scipy,Eric89GXL/scipy,Gillu13/scipy,person142/scipy,chatcannon/scipy,fernand/scipy,tylerjereddy/scipy,argriffing/scipy,ales-erjavec/scipy,Newman101/scipy,cpaulik/scipy,minhlongdo/scipy,befelix/scipy,woodscn/scipy,petebachant/scipy,vberaudi/scipy,vanpact/scipy