[Doc] Improvements to ddl.sgl Privileges Section and Glossary

Hey,

In a nearby user complaint email [1] some missing information regarding
ownership reassignment came to light. I took that and went a bit further
to add what I felt was further missing information and context for how the
privilege system is designed. I've tried to formalize and label existing
concepts a bit and updated the glossary accordingly.

The attached is a partial rewrite of the patch on the linked post after
those comments were taken into consideration.

The new glossary entry for privileges defines various qualifications of the
term that are used in the new prose. I've marked up each of the variants
and point them all back to the main entry. I didn't try to incorporate
those terms, or even really look, anywhere else in the documentation. If
the general idea is accepted that kind of work can be done as a follow-up.

David J.

[1]
https://www.postgresql.org/message-id/d294818d12280f6223ddf169ab5454927f5186b6.camel%40cybertec.at