DOC: More accurate flattop coefficients and docs
DOC: More accurate flattop coefficients and docs

Coefficients taken from the D’Antona reference (though the improved
accuracy makes little practical difference).

Fixes #4234

jakevdp/scipy,pschella/scipy,anntzer/scipy,woodscn/scipy,arokem/scipy,matthewalbani/scipy,e-q/scipy,nonhermitian/scipy,dominicelse/scipy,person142/scipy,aarchiba/scipy,aeklant/scipy,pschella/scipy,aeklant/scipy,pizzathief/scipy,jjhelmus/scipy,matthew-brett/scipy,lhilt/scipy,josephcslater/scipy,mdhaber/scipy,anntzer/scipy,ilayn/scipy,andyfaff/scipy,anielsen001/scipy,gdooper/scipy,endolith/scipy,Stefan-Endres/scipy,scipy/scipy,tylerjereddy/scipy,maniteja123/scipy,perimosocordiae/scipy,maniteja123/scipy,mikebenfield/scipy,jjhelmus/scipy,pbrod/scipy,sriki18/scipy,scipy/scipy,pschella/scipy,jjhelmus/scipy,grlee77/scipy,lhilt/scipy,josephcslater/scipy,mdhaber/scipy,nonhermitian/scipy,andyfaff/scipy,pizzathief/scipy,matthew-brett/scipy,surhudm/scipy,maniteja123/scipy,nonhermitian/scipy,woodscn/scipy,nonhermitian/scipy,jor-/scipy,mikebenfield/scipy,aarchiba/scipy,aeklant/scipy,apbard/scipy,person142/scipy,josephcslater/scipy,endolith/scipy,apbard/scipy,Eric89GXL/scipy,aarchiba/scipy,vigna/scipy,anielsen001/scipy,matthewalbani/scipy,anielsen001/scipy,anielsen001/scipy,anntzer/scipy,jamestwebber/scipy,dominicelse/scipy,woodscn/scipy,rgommers/scipy,e-q/scipy,Eric89GXL/scipy,jor-/scipy,gdooper/scipy,rgommers/scipy,surhudm/scipy,e-q/scipy,tylerjereddy/scipy,jor-/scipy,pbrod/scipy,Eric89GXL/scipy,anielsen001/scipy,tylerjereddy/scipy,pyramania/scipy,lhilt/scipy,nmayorov/scipy,mikebenfield/scipy,endolith/scipy,matthewalbani/scipy,ilayn/scipy,kalvdans/scipy,endolith/scipy,perimosocordiae/scipy,woodscn/scipy,gfyoung/scipy,mdhaber/scipy,apbard/scipy,perimosocordiae/scipy,WarrenWeckesser/scipy,nonhermitian/scipy,kalvdans/scipy,grlee77/scipy,andyfaff/scipy,lhilt/scipy,anntzer/scipy,aarchiba/scipy,pschella/scipy,surhudm/scipy,andyfaff/scipy,bkendzior/scipy,WarrenWeckesser/scipy,pbrod/scipy,arokem/scipy,josephcslater/scipy,Stefan-Endres/scipy,zerothi/scipy,gertingold/scipy,mdhaber/scipy,scipy/scipy,aeklant/scipy,jakevdp/scipy,anntzer/scipy,gdooper/scipy,gdooper/scipy,sriki18/scipy,jor-/scipy,maniteja123/scipy,sriki18/scipy,mikebenfield/scipy,surhudm/scipy,maniteja123/scipy,zerothi/scipy,befelix/scipy,nmayorov/scipy,ilayn/scipy,arokem/scipy,vigna/scipy,pyramania/scipy,perimosocordiae/scipy,scipy/scipy,tylerjereddy/scipy,Stefan-Endres/scipy,endolith/scipy,mikebenfield/scipy,matthewalbani/scipy,scipy/scipy,Stefan-Endres/scipy,jakevdp/scipy,grlee77/scipy,vigna/scipy,bkendzior/scipy,pbrod/scipy,gfyoung/scipy,jamestwebber/scipy,mdhaber/scipy,Eric89GXL/scipy,WarrenWeckesser/scipy,lhilt/scipy,perimosocordiae/scipy,zerothi/scipy,zerothi/scipy,jjhelmus/scipy,ilayn/scipy,zerothi/scipy,WarrenWeckesser/scipy,gfyoung/scipy,pyramania/scipy,Eric89GXL/scipy,befelix/scipy,e-q/scipy,rgommers/scipy,arokem/scipy,sriki18/scipy,nmayorov/scipy,matthew-brett/scipy,tylerjereddy/scipy,sriki18/scipy,pschella/scipy,woodscn/scipy,surhudm/scipy,rgommers/scipy,person142/scipy,apbard/scipy,gertingold/scipy,jakevdp/scipy,kalvdans/scipy,bkendzior/scipy,grlee77/scipy,anntzer/scipy,josephcslater/scipy,befelix/scipy,Eric89GXL/scipy,sriki18/scipy,maniteja123/scipy,WarrenWeckesser/scipy,gertingold/scipy,befelix/scipy,Stefan-Endres/scipy,WarrenWeckesser/scipy,gdooper/scipy,matthewalbani/scipy,pyramania/scipy,kalvdans/scipy,matthew-brett/scipy,person142/scipy,pizzathief/scipy,gertingold/scipy,jamestwebber/scipy,kalvdans/scipy,aeklant/scipy,gfyoung/scipy,rgommers/scipy,apbard/scipy,anielsen001/scipy,dominicelse/scipy,jamestwebber/scipy,dominicelse/scipy,befelix/scipy,jor-/scipy,endolith/scipy,e-q/scipy,Stefan-Endres/scipy,pizzathief/scipy,perimosocordiae/scipy,matthew-brett/scipy,nmayorov/scipy,pbrod/scipy,jjhelmus/scipy,surhudm/scipy,gertingold/scipy,person142/scipy,vigna/scipy,andyfaff/scipy,bkendzior/scipy,mdhaber/scipy,gfyoung/scipy,zerothi/scipy,nmayorov/scipy,pyramania/scipy,scipy/scipy,bkendzior/scipy,jamestwebber/scipy,jakevdp/scipy,ilayn/scipy,vigna/scipy,woodscn/scipy,grlee77/scipy,ilayn/scipy,andyfaff/scipy,arokem/scipy,pizzathief/scipy,pbrod/scipy,dominicelse/scipy,aarchiba/scipy