DOC: documents parameter `limit` for function `integrate.quad_vec`.
DOC: documents parameter `limit` for function `integrate.quad_vec`.

Description now aligns with that of `integrate.quad`. Closes #15415.

scipy/scipy,perimosocordiae/scipy,mdhaber/scipy,endolith/scipy,WarrenWeckesser/scipy,WarrenWeckesser/scipy,rgommers/scipy,anntzer/scipy,rgommers/scipy,andyfaff/scipy,ilayn/scipy,WarrenWeckesser/scipy,ilayn/scipy,anntzer/scipy,vigna/scipy,ilayn/scipy,rgommers/scipy,WarrenWeckesser/scipy,andyfaff/scipy,endolith/scipy,Stefan-Endres/scipy,perimosocordiae/scipy,perimosocordiae/scipy,scipy/scipy,scipy/scipy,mdhaber/scipy,Stefan-Endres/scipy,anntzer/scipy,scipy/scipy,WarrenWeckesser/scipy,endolith/scipy,endolith/scipy,vigna/scipy,mdhaber/scipy,Stefan-Endres/scipy,andyfaff/scipy,Stefan-Endres/scipy,mdhaber/scipy,andyfaff/scipy,ilayn/scipy,mdhaber/scipy,endolith/scipy,scipy/scipy,endolith/scipy,vigna/scipy,anntzer/scipy,rgommers/scipy,anntzer/scipy,anntzer/scipy,Stefan-Endres/scipy,vigna/scipy,WarrenWeckesser/scipy,Stefan-Endres/scipy,ilayn/scipy,rgommers/scipy,mdhaber/scipy,andyfaff/scipy,andyfaff/scipy,perimosocordiae/scipy,vigna/scipy,scipy/scipy,perimosocordiae/scipy,ilayn/scipy,perimosocordiae/scipy