pgsql: Glossary: improve definition of "relation"

Glossary: improve definition of "relation"

Define the more general term first, then the Postgres-specific meaning.

Wording from Tom Lane.

Discussion: https://postgr.es/m/CACJufxEZ48toGH0Em_6vdsT57Y3L8pLF=DZCQ_gCii6=C3MeXw@mail.gmail.com

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/16a9165ce4a45f11aa042e12ab62be8ce6b2b93b

Modified Files
--------------
doc/src/sgml/glossary.sgml | 27 +++++++++++++++------------
1 file changed, 15 insertions(+), 12 deletions(-)