Clarify documentation for lazy_loaded_from
Clarify documentation for lazy_loaded_from

This attribute was never available in conjunction with
mapper options, and additionally the use of baked queries
in the lazy loader strategy prevent this attribute from
being reliably available within the before_compile hook
unless baked queries are turned off entirely.

Fixes: #5109
Change-Id: I5fa12c68463d20475e7470647a784efe846b8af7

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