Add doc note for multiple table mapping
Add doc note for multiple table mapping

When mapping to a construct like OUTER JOIN, an UPDATE from the
ORM expects that all involved tables have a row already present;
document this as well as a potential workaround.

Fixes: #4927
Change-Id: Ie99f9f53cea33d9df2513f384a7c68676b197fb7

graingert/sqlalchemy,monetate/sqlalchemy,j5int/sqlalchemy,zzzeek/sqlalchemy,sqlalchemy/sqlalchemy,j5int/sqlalchemy,graingert/sqlalchemy,monetate/sqlalchemy