DOC: special: add examples to `hyp2f1`
DOC: special: add examples to `hyp2f1`

At this point all the hypergeometric functions have complete
docstrings, so this closes gh-6966.

matthew-brett/scipy,rgommers/scipy,zerothi/scipy,gertingold/scipy,Eric89GXL/scipy,scipy/scipy,WarrenWeckesser/scipy,andyfaff/scipy,vigna/scipy,jamestwebber/scipy,arokem/scipy,gertingold/scipy,ilayn/scipy,e-q/scipy,grlee77/scipy,tylerjereddy/scipy,jamestwebber/scipy,vigna/scipy,gertingold/scipy,endolith/scipy,aarchiba/scipy,ilayn/scipy,aarchiba/scipy,endolith/scipy,e-q/scipy,jor-/scipy,Stefan-Endres/scipy,perimosocordiae/scipy,endolith/scipy,rgommers/scipy,scipy/scipy,tylerjereddy/scipy,zerothi/scipy,gertingold/scipy,mdhaber/scipy,perimosocordiae/scipy,andyfaff/scipy,lhilt/scipy,aeklant/scipy,ilayn/scipy,rgommers/scipy,scipy/scipy,matthew-brett/scipy,grlee77/scipy,lhilt/scipy,perimosocordiae/scipy,endolith/scipy,pizzathief/scipy,andyfaff/scipy,tylerjereddy/scipy,person142/scipy,Eric89GXL/scipy,matthew-brett/scipy,perimosocordiae/scipy,arokem/scipy,matthew-brett/scipy,tylerjereddy/scipy,endolith/scipy,e-q/scipy,ilayn/scipy,zerothi/scipy,scipy/scipy,jamestwebber/scipy,aarchiba/scipy,zerothi/scipy,Stefan-Endres/scipy,pizzathief/scipy,ilayn/scipy,jor-/scipy,WarrenWeckesser/scipy,grlee77/scipy,anntzer/scipy,nmayorov/scipy,mdhaber/scipy,Eric89GXL/scipy,jor-/scipy,lhilt/scipy,aeklant/scipy,scipy/scipy,anntzer/scipy,anntzer/scipy,gertingold/scipy,perimosocordiae/scipy,nmayorov/scipy,e-q/scipy,nmayorov/scipy,arokem/scipy,anntzer/scipy,WarrenWeckesser/scipy,tylerjereddy/scipy,Stefan-Endres/scipy,Eric89GXL/scipy,jamestwebber/scipy,Eric89GXL/scipy,arokem/scipy,pizzathief/scipy,Stefan-Endres/scipy,andyfaff/scipy,mdhaber/scipy,Stefan-Endres/scipy,grlee77/scipy,rgommers/scipy,WarrenWeckesser/scipy,anntzer/scipy,aarchiba/scipy,aarchiba/scipy,person142/scipy,person142/scipy,pizzathief/scipy,rgommers/scipy,ilayn/scipy,zerothi/scipy,arokem/scipy,matthew-brett/scipy,nmayorov/scipy,lhilt/scipy,lhilt/scipy,anntzer/scipy,vigna/scipy,aeklant/scipy,endolith/scipy,vigna/scipy,person142/scipy,WarrenWeckesser/scipy,Stefan-Endres/scipy,aeklant/scipy,jamestwebber/scipy,pizzathief/scipy,vigna/scipy,Eric89GXL/scipy,mdhaber/scipy,WarrenWeckesser/scipy,aeklant/scipy,andyfaff/scipy,mdhaber/scipy,jor-/scipy,e-q/scipy,mdhaber/scipy,jor-/scipy,person142/scipy,nmayorov/scipy,grlee77/scipy,zerothi/scipy,scipy/scipy,andyfaff/scipy,perimosocordiae/scipy