doc: improve description of privileges for MERGE and update glossary.

doc: improve description of privileges for MERGE and update glossary.

On the MERGE page, the description of the privileges required could be
taken to imply that the SELECT privilege is required on all columns of
the data source, whereas actually it is only required on the columns
referred to by conditions or expressions in the MERGE command. Re-word
it to make that a little clearer, and mention expressions as well as
conditions.

Also, add a glossary entry for MERGE, and nearby on the glossary page,
mention MERGE in the list of commands that cannot update a
materialized view.

Noted by Jian He. Patch by me, reviewed by Jian He.

Discussion: https://postgr.es/m/CACJufxHuSoRXKwr0MtSFLXuT2nFVWcVfEWhxg7qdP9h%2Bs3a%2BUw%40mail.gmail.com