doc: New glossary entries

doc: New glossary entries

Add:

- "Restartpoint"
- "Log sequence number"

"LSN" was already listed in the Acronyms appendix, but it is more
suitable as a glossary entry, so move it there and have the acronyms
entry link into the glossary.

Also turn on DocBook parameter glossentry.show.acronym to show
acronyms for glossary entries, which is being used here.

Reviewed-by: Daniel Gustafsson <daniel@yesql.se>
Reviewed-by: Alvaro Herrera <alvherre@alvh.no-ip.org>
Discussion: https://www.postgresql.org/message-id/flat/60915312-62cd-9c94-0d94-556023ece45f%40enterprisedb.com