REASSIGN OWNED: handle shared objects, too

REASSIGN OWNED: handle shared objects, too

Give away ownership of shared objects (databases, tablespaces) along
with local objects, per original code intention.  Try to make the
documentation clearer, too.

Per discussion about DROP OWNED's brokenness, in bug #7748.

This is not backpatched because it'd require some refactoring of the
ALTER/SET OWNER code for databases and tablespaces.