DOC: Include nextafter and spacing function in documentation.
DOC: Include nextafter and spacing function in documentation.

Fixes #8831

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