Various small doc improvements; plpgsql, schemas, permissions, oidvector

Hi,

I have made various, mostly unrelated to each other,
small improvements to the documentation. These
are usually in the areas of plpgsql, schemas, and permissions.
Most change 1 lines, but some supply short overviews.

"Short" is subjective, so if these need to be
broken into different threads or different
commitfest entries let me know. I'm starting
simple and submitting a single patch.

Attached: various_doc_patches_v1.patch

Regards,

Karl <kop(at)karlpinc(dot)com>
Free Software: "You don't pay back, you pay forward."
-- Robert A. Heinlein