Improve documentation for pg_largeobject changes.

Rewrite the documentation in more idiomatic English, and in the process make
it somewhat more succinct.  Move the discussion of specific large object
privileges out of the "server-side functions" section, where it certainly
doesn't belong, and into "implementation features".  That might not be
exactly right either, but it doesn't seem worth creating a new section for
this amount of information. Fix a few spelling and layout problems, too.