added docs to case() illusrtating usage of `literal_column()`, can't implement #809 directly
added docs to case() illusrtating usage of `literal_column()`, can't implement #809 directly

halfcrazy/sqlalchemy,epa/sqlalchemy,elelianghh/sqlalchemy,Akrog/sqlalchemy,robin900/sqlalchemy,alex/sqlalchemy,dstufft/sqlalchemy,pdufour/sqlalchemy,wujuguang/sqlalchemy,brianv0/sqlalchemy,EvaSDK/sqlalchemy,graingert/sqlalchemy,alex/sqlalchemy,graingert/sqlalchemy,brianv0/sqlalchemy,Akrog/sqlalchemy,hsum/sqlalchemy,wfxiang08/sqlalchemy,Cito/sqlalchemy,dstufft/sqlalchemy,olemis/sqlalchemy,wujuguang/sqlalchemy,bootandy/sqlalchemy,elelianghh/sqlalchemy,itkovian/sqlalchemy,WinterNis/sqlalchemy,wfxiang08/sqlalchemy,bdupharm/sqlalchemy,bdupharm/sqlalchemy,monetate/sqlalchemy,j5int/sqlalchemy,Cito/sqlalchemy,davidjb/sqlalchemy,EvaSDK/sqlalchemy,ThiefMaster/sqlalchemy,davidfraser/sqlalchemy,inspirehep/sqlalchemy,276361270/sqlalchemy,sqlalchemy/sqlalchemy,davidfraser/sqlalchemy,WinterNis/sqlalchemy,sandan/sqlalchemy,itkovian/sqlalchemy,monetate/sqlalchemy,robin900/sqlalchemy,ThiefMaster/sqlalchemy,j5int/sqlalchemy,alex/sqlalchemy,276361270/sqlalchemy,davidjb/sqlalchemy,inspirehep/sqlalchemy,olemis/sqlalchemy,bootandy/sqlalchemy,halfcrazy/sqlalchemy,zzzeek/sqlalchemy,pdufour/sqlalchemy,hsum/sqlalchemy,Cito/sqlalchemy,epa/sqlalchemy,sandan/sqlalchemy