DOC: Document isnat addition and related warning changes
DOC: Document isnat addition and related warning changes

mattip/numpy,numpy/numpy,simongibbons/numpy,pbrod/numpy,ahaldane/numpy,madphysicist/numpy,madphysicist/numpy,rgommers/numpy,shoyer/numpy,simongibbons/numpy,mhvk/numpy,tynn/numpy,MSeifert04/numpy,anntzer/numpy,MSeifert04/numpy,endolith/numpy,WarrenWeckesser/numpy,charris/numpy,pizzathief/numpy,ahaldane/numpy,mattip/numpy,jakirkham/numpy,Eric89GXL/numpy,MSeifert04/numpy,mattip/numpy,pbrod/numpy,abalkin/numpy,jakirkham/numpy,seberg/numpy,solarjoe/numpy,jakirkham/numpy,madphysicist/numpy,Eric89GXL/numpy,gfyoung/numpy,seberg/numpy,pdebuyl/numpy,gfyoung/numpy,b-carter/numpy,Eric89GXL/numpy,bringingheavendown/numpy,jakirkham/numpy,WarrenWeckesser/numpy,jorisvandenbossche/numpy,anntzer/numpy,pdebuyl/numpy,endolith/numpy,endolith/numpy,charris/numpy,simongibbons/numpy,grlee77/numpy,anntzer/numpy,endolith/numpy,jorisvandenbossche/numpy,shoyer/numpy,shoyer/numpy,pdebuyl/numpy,grlee77/numpy,pbrod/numpy,tynn/numpy,Eric89GXL/numpy,WarrenWeckesser/numpy,tynn/numpy,numpy/numpy,charris/numpy,jakirkham/numpy,rgommers/numpy,numpy/numpy,abalkin/numpy,jorisvandenbossche/numpy,pizzathief/numpy,pdebuyl/numpy,mhvk/numpy,shoyer/numpy,seberg/numpy,madphysicist/numpy,solarjoe/numpy,mhvk/numpy,mhvk/numpy,MSeifert04/numpy,rgommers/numpy,pizzathief/numpy,bringingheavendown/numpy,grlee77/numpy,simongibbons/numpy,pizzathief/numpy,anntzer/numpy,WarrenWeckesser/numpy,simongibbons/numpy,rgommers/numpy,ahaldane/numpy,grlee77/numpy,pbrod/numpy,jorisvandenbossche/numpy,mhvk/numpy,shoyer/numpy,solarjoe/numpy,pbrod/numpy,MSeifert04/numpy,numpy/numpy,jorisvandenbossche/numpy,b-carter/numpy,bringingheavendown/numpy,b-carter/numpy,ahaldane/numpy,abalkin/numpy,mattip/numpy,gfyoung/numpy,ahaldane/numpy,madphysicist/numpy,WarrenWeckesser/numpy,pizzathief/numpy,charris/numpy,seberg/numpy,grlee77/numpy