DOC: fix documentation for scipy.optimize.brenth
DOC: fix documentation for scipy.optimize.brenth

See #14390. This commit adds a proper reference for the brenth method,
along with the necessary citation.

matthew-brett/scipy,mdhaber/scipy,tylerjereddy/scipy,ilayn/scipy,ilayn/scipy,matthew-brett/scipy,tylerjereddy/scipy,matthew-brett/scipy,perimosocordiae/scipy,Stefan-Endres/scipy,ilayn/scipy,Stefan-Endres/scipy,matthew-brett/scipy,mdhaber/scipy,rgommers/scipy,anntzer/scipy,scipy/scipy,rgommers/scipy,anntzer/scipy,endolith/scipy,endolith/scipy,tylerjereddy/scipy,zerothi/scipy,anntzer/scipy,zerothi/scipy,mdhaber/scipy,scipy/scipy,ilayn/scipy,andyfaff/scipy,matthew-brett/scipy,vigna/scipy,mdhaber/scipy,mdhaber/scipy,mdhaber/scipy,anntzer/scipy,scipy/scipy,perimosocordiae/scipy,perimosocordiae/scipy,vigna/scipy,zerothi/scipy,Stefan-Endres/scipy,tylerjereddy/scipy,Stefan-Endres/scipy,endolith/scipy,WarrenWeckesser/scipy,perimosocordiae/scipy,scipy/scipy,vigna/scipy,zerothi/scipy,rgommers/scipy,zerothi/scipy,WarrenWeckesser/scipy,Stefan-Endres/scipy,endolith/scipy,scipy/scipy,perimosocordiae/scipy,ilayn/scipy,WarrenWeckesser/scipy,anntzer/scipy,tylerjereddy/scipy,andyfaff/scipy,rgommers/scipy,zerothi/scipy,endolith/scipy,andyfaff/scipy,anntzer/scipy,WarrenWeckesser/scipy,endolith/scipy,perimosocordiae/scipy,rgommers/scipy,ilayn/scipy,vigna/scipy,WarrenWeckesser/scipy,WarrenWeckesser/scipy,andyfaff/scipy,andyfaff/scipy,Stefan-Endres/scipy,andyfaff/scipy,scipy/scipy,vigna/scipy