DOC: add missing details to linalg.lstsq docstring
DOC: add missing details to linalg.lstsq docstring

It turns out that lstsq also minimizes the 2-norm of x when a is
rank-deficient. I found that by searching the documentation for the
LAPACK library, which is the current implementation of lstsq (as of Dec
2020). Ref: https://www.netlib.org/lapack/lug/node27.html

madphysicist/numpy,rgommers/numpy,pbrod/numpy,simongibbons/numpy,simongibbons/numpy,anntzer/numpy,simongibbons/numpy,charris/numpy,jakirkham/numpy,charris/numpy,mattip/numpy,seberg/numpy,rgommers/numpy,pbrod/numpy,mhvk/numpy,rgommers/numpy,madphysicist/numpy,simongibbons/numpy,mhvk/numpy,numpy/numpy,pdebuyl/numpy,numpy/numpy,anntzer/numpy,rgommers/numpy,mhvk/numpy,anntzer/numpy,numpy/numpy,jakirkham/numpy,endolith/numpy,mhvk/numpy,mattip/numpy,anntzer/numpy,endolith/numpy,jakirkham/numpy,endolith/numpy,seberg/numpy,seberg/numpy,mhvk/numpy,madphysicist/numpy,madphysicist/numpy,numpy/numpy,pbrod/numpy,pdebuyl/numpy,pbrod/numpy,charris/numpy,mattip/numpy,jakirkham/numpy,jakirkham/numpy,endolith/numpy,seberg/numpy,pbrod/numpy,charris/numpy,pdebuyl/numpy,madphysicist/numpy,pdebuyl/numpy,mattip/numpy,simongibbons/numpy