Add documentation not about ``_unit_representation``
Add documentation not about ``_unit_representation``

funbaker/astropy,pllim/astropy,StuartLittlefair/astropy,DougBurke/astropy,astropy/astropy,joergdietrich/astropy,mhvk/astropy,mhvk/astropy,joergdietrich/astropy,aleksandr-bakanov/astropy,stargaser/astropy,joergdietrich/astropy,larrybradley/astropy,saimn/astropy,tbabej/astropy,MSeifert04/astropy,astropy/astropy,astropy/astropy,saimn/astropy,AustereCuriosity/astropy,mhvk/astropy,MSeifert04/astropy,larrybradley/astropy,bsipocz/astropy,saimn/astropy,mhvk/astropy,AustereCuriosity/astropy,DougBurke/astropy,AustereCuriosity/astropy,stargaser/astropy,funbaker/astropy,funbaker/astropy,bsipocz/astropy,mhvk/astropy,dhomeier/astropy,kelle/astropy,lpsinger/astropy,AustereCuriosity/astropy,pllim/astropy,dhomeier/astropy,tbabej/astropy,MSeifert04/astropy,astropy/astropy,tbabej/astropy,StuartLittlefair/astropy,dhomeier/astropy,funbaker/astropy,MSeifert04/astropy,lpsinger/astropy,dhomeier/astropy,kelle/astropy,tbabej/astropy,kelle/astropy,saimn/astropy,pllim/astropy,aleksandr-bakanov/astropy,dhomeier/astropy,larrybradley/astropy,larrybradley/astropy,saimn/astropy,DougBurke/astropy,stargaser/astropy,joergdietrich/astropy,stargaser/astropy,kelle/astropy,bsipocz/astropy,lpsinger/astropy,pllim/astropy,AustereCuriosity/astropy,kelle/astropy,pllim/astropy,lpsinger/astropy,lpsinger/astropy,bsipocz/astropy,astropy/astropy,larrybradley/astropy,aleksandr-bakanov/astropy,StuartLittlefair/astropy,joergdietrich/astropy,StuartLittlefair/astropy,DougBurke/astropy,tbabej/astropy,StuartLittlefair/astropy,aleksandr-bakanov/astropy