Add docs on group visibility
Add docs on group visibility

[ci skip]

axilleas/gitlabhq,screenpages/gitlabhq,screenpages/gitlabhq,Soullivaneuh/gitlabhq,allysonbarros/gitlabhq,daiyu/gitlab-zh,openwide-java/gitlabhq,jirutka/gitlabhq,mmkassem/gitlabhq,darkrasid/gitlabhq,htve/GitlabForChinese,t-zuehlsdorff/gitlabhq,stoplightio/gitlabhq,dplarson/gitlabhq,dwrensha/gitlabhq,Soullivaneuh/gitlabhq,axilleas/gitlabhq,jirutka/gitlabhq,dwrensha/gitlabhq,screenpages/gitlabhq,Soullivaneuh/gitlabhq,larryli/gitlabhq,SVArago/gitlabhq,icedwater/gitlabhq,martijnvermaat/gitlabhq,mr-dxdy/gitlabhq,mr-dxdy/gitlabhq,mr-dxdy/gitlabhq,LUMC/gitlabhq,shinexiao/gitlabhq,openwide-java/gitlabhq,icedwater/gitlabhq,LUMC/gitlabhq,SVArago/gitlabhq,jirutka/gitlabhq,mmkassem/gitlabhq,Soullivaneuh/gitlabhq,icedwater/gitlabhq,SVArago/gitlabhq,dwrensha/gitlabhq,mmkassem/gitlabhq,dplarson/gitlabhq,stoplightio/gitlabhq,dplarson/gitlabhq,SVArago/gitlabhq,LUMC/gitlabhq,shinexiao/gitlabhq,allysonbarros/gitlabhq,daiyu/gitlab-zh,larryli/gitlabhq,martijnvermaat/gitlabhq,icedwater/gitlabhq,mmkassem/gitlabhq,iiet/iiet-git,iiet/iiet-git,larryli/gitlabhq,daiyu/gitlab-zh,larryli/gitlabhq,martijnvermaat/gitlabhq,darkrasid/gitlabhq,dwrensha/gitlabhq,htve/GitlabForChinese,htve/GitlabForChinese,iiet/iiet-git,allysonbarros/gitlabhq,t-zuehlsdorff/gitlabhq,daiyu/gitlab-zh,openwide-java/gitlabhq,jirutka/gitlabhq,dreampet/gitlab,darkrasid/gitlabhq,stoplightio/gitlabhq,darkrasid/gitlabhq,martijnvermaat/gitlabhq,stoplightio/gitlabhq,dreampet/gitlab,t-zuehlsdorff/gitlabhq,shinexiao/gitlabhq,htve/GitlabForChinese,axilleas/gitlabhq,iiet/iiet-git,LUMC/gitlabhq,openwide-java/gitlabhq,allysonbarros/gitlabhq,axilleas/gitlabhq,shinexiao/gitlabhq,dplarson/gitlabhq,t-zuehlsdorff/gitlabhq,dreampet/gitlab,dreampet/gitlab,mr-dxdy/gitlabhq,screenpages/gitlabhq