Further improve documentation of the role-dropping process.

Further improve documentation of the role-dropping process.

In commit 1ea0c73c2 I added a section to user-manag.sgml about how to drop
roles that own objects; but as pointed out by Stephen Frost, I neglected
that shared objects (databases or tablespaces) may need special treatment.
Fix that.  Back-patch to supported versions, like the previous patch.