NEP: clarify the purpose of "types" in __array_function__
NEP: clarify the purpose of "types" in __array_function__

Per our discussion on the mailing list:
https://mail.python.org/pipermail/numpy-discussion/2018-November/078911.html

seberg/numpy,abalkin/numpy,simongibbons/numpy,pbrod/numpy,mattip/numpy,pizzathief/numpy,mhvk/numpy,grlee77/numpy,grlee77/numpy,jorisvandenbossche/numpy,jakirkham/numpy,endolith/numpy,endolith/numpy,anntzer/numpy,charris/numpy,anntzer/numpy,anntzer/numpy,endolith/numpy,MSeifert04/numpy,pdebuyl/numpy,seberg/numpy,shoyer/numpy,madphysicist/numpy,charris/numpy,WarrenWeckesser/numpy,jakirkham/numpy,charris/numpy,seberg/numpy,pbrod/numpy,grlee77/numpy,pizzathief/numpy,mhvk/numpy,WarrenWeckesser/numpy,seberg/numpy,ahaldane/numpy,pdebuyl/numpy,anntzer/numpy,mattip/numpy,MSeifert04/numpy,charris/numpy,rgommers/numpy,shoyer/numpy,MSeifert04/numpy,numpy/numpy,WarrenWeckesser/numpy,mattip/numpy,pizzathief/numpy,jakirkham/numpy,simongibbons/numpy,jakirkham/numpy,pdebuyl/numpy,MSeifert04/numpy,rgommers/numpy,WarrenWeckesser/numpy,mhvk/numpy,numpy/numpy,mhvk/numpy,pbrod/numpy,mattip/numpy,madphysicist/numpy,ahaldane/numpy,numpy/numpy,WarrenWeckesser/numpy,abalkin/numpy,pbrod/numpy,ahaldane/numpy,jorisvandenbossche/numpy,pizzathief/numpy,jorisvandenbossche/numpy,madphysicist/numpy,shoyer/numpy,shoyer/numpy,simongibbons/numpy,simongibbons/numpy,ahaldane/numpy,MSeifert04/numpy,pdebuyl/numpy,grlee77/numpy,simongibbons/numpy,madphysicist/numpy,mhvk/numpy,endolith/numpy,grlee77/numpy,jakirkham/numpy,pbrod/numpy,rgommers/numpy,jorisvandenbossche/numpy,numpy/numpy,pizzathief/numpy,shoyer/numpy,abalkin/numpy,ahaldane/numpy,madphysicist/numpy,rgommers/numpy,jorisvandenbossche/numpy