DOC: special: complete the docstrings of Chi-square functions
DOC: special: complete the docstrings of Chi-square functions

In particular, add:

- parameters/returns sections
- see also sections
- examples
- references

for `chdtr`, `chdtrc`, `chdtri`, and `chdtriv`.

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