add a couple expository notes to docs; fix a couple rst buglets.  use modern mapper style (i.e., no need to add it to klass as an attribute).
add a couple expository notes to docs; fix a couple rst buglets.  use modern mapper style (i.e., no need to add it to klass as an attribute).

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