DOC: special: add to the docstring of `gammaln` (#10742)
DOC: special: add to the docstring of `gammaln` (#10742)

In particular:

- add to the definition
- standardize the parameters/returns section
- cross reference `math.lgamma`
- add examples
aarchiba/scipy,nmayorov/scipy,perimosocordiae/scipy,mdhaber/scipy,andyfaff/scipy,andyfaff/scipy,jor-/scipy,pizzathief/scipy,ilayn/scipy,tylerjereddy/scipy,rgommers/scipy,aeklant/scipy,anntzer/scipy,jamestwebber/scipy,endolith/scipy,jor-/scipy,aeklant/scipy,Stefan-Endres/scipy,nmayorov/scipy,scipy/scipy,ilayn/scipy,ilayn/scipy,tylerjereddy/scipy,Stefan-Endres/scipy,mdhaber/scipy,mdhaber/scipy,jamestwebber/scipy,andyfaff/scipy,WarrenWeckesser/scipy,Stefan-Endres/scipy,nmayorov/scipy,zerothi/scipy,anntzer/scipy,aarchiba/scipy,endolith/scipy,anntzer/scipy,grlee77/scipy,tylerjereddy/scipy,andyfaff/scipy,tylerjereddy/scipy,e-q/scipy,arokem/scipy,nmayorov/scipy,pizzathief/scipy,rgommers/scipy,mdhaber/scipy,ilayn/scipy,jamestwebber/scipy,anntzer/scipy,matthew-brett/scipy,scipy/scipy,aarchiba/scipy,jor-/scipy,person142/scipy,WarrenWeckesser/scipy,arokem/scipy,WarrenWeckesser/scipy,Stefan-Endres/scipy,endolith/scipy,zerothi/scipy,Stefan-Endres/scipy,tylerjereddy/scipy,aarchiba/scipy,zerothi/scipy,aeklant/scipy,Eric89GXL/scipy,endolith/scipy,mdhaber/scipy,matthew-brett/scipy,rgommers/scipy,e-q/scipy,perimosocordiae/scipy,rgommers/scipy,jamestwebber/scipy,endolith/scipy,perimosocordiae/scipy,pizzathief/scipy,matthew-brett/scipy,matthew-brett/scipy,mdhaber/scipy,jor-/scipy,person142/scipy,e-q/scipy,endolith/scipy,vigna/scipy,scipy/scipy,arokem/scipy,grlee77/scipy,scipy/scipy,aarchiba/scipy,vigna/scipy,grlee77/scipy,grlee77/scipy,nmayorov/scipy,andyfaff/scipy,scipy/scipy,vigna/scipy,WarrenWeckesser/scipy,grlee77/scipy,arokem/scipy,aeklant/scipy,Eric89GXL/scipy,jamestwebber/scipy,zerothi/scipy,ilayn/scipy,pizzathief/scipy,Eric89GXL/scipy,vigna/scipy,aeklant/scipy,ilayn/scipy,Eric89GXL/scipy,perimosocordiae/scipy,matthew-brett/scipy,Eric89GXL/scipy,rgommers/scipy,WarrenWeckesser/scipy,WarrenWeckesser/scipy,person142/scipy,vigna/scipy,anntzer/scipy,jor-/scipy,arokem/scipy,scipy/scipy,perimosocordiae/scipy,perimosocordiae/scipy,e-q/scipy,Eric89GXL/scipy,person142/scipy,pizzathief/scipy,Stefan-Endres/scipy,anntzer/scipy,zerothi/scipy,person142/scipy,e-q/scipy,zerothi/scipy,andyfaff/scipy