add hooks/docs for automap w/ multiple schemas

add hooks/docs for automap w/ multiple schemas

Added new feature to :class:`.Automap` for autoload of classes across
multiple schemas which may have overlapping names, by providing both a
:paramref:`.Automap.prepare.modulename_for_class` parameter as well as a
new collection :attr:`.AutomapBase.by_module`, which stores a dot-separated
namespace of module names linked to classes.

Fixes: #5145
Change-Id: I735fecaacdfc267f1f901d76c2b3880e48f5969d