pgsql: doc: Add acronym and glossary term for Access Method

doc: Add acronym and glossary term for Access Method

AM was used throughout the documentation to denote Access Method, but
the acronym was not described. This adds an acronym entry as well as
a glossary term which the acronym links to. Each page which describe
AMs have the first occurrence with <acronym> markup.

Reported-by: alaa(dot)attya91(at)gmail(dot)com
Reviewed-by: Alvaro Herrera <alvherre(at)alvh(dot)no-ip(dot)org>
Discussion: https://postgr.es/m/169974408805.398198.6927340566912872957@wrigleys.postgresql.org

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/68387f9bdc82a56bf56945da3883a993bc40e2a5

Modified Files
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doc/src/sgml/acronyms.sgml | 9 +++++++++
doc/src/sgml/btree.sgml | 2 +-
doc/src/sgml/glossary.sgml | 15 +++++++++++++++
doc/src/sgml/indexam.sgml | 2 +-
doc/src/sgml/tableam.sgml | 2 +-
5 files changed, 27 insertions(+), 3 deletions(-)