pgsql: doc: add section about heap-only tuples (HOT)

doc:  add section about heap-only tuples (HOT)

Reported-by: Jonathan S. Katz

Discussion: https://postgr.es/m/c59ffbd5-96ac-a5a5-a401-14f627ca1405@postgresql.org

Backpatch-through: 11

Branch
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REL_15_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/4445461cdff3580a2a698e89f66efd467f1bb5df

Modified Files
--------------
doc/src/sgml/acronyms.sgml         |  4 +--
doc/src/sgml/btree.sgml            |  3 +-
doc/src/sgml/catalogs.sgml         |  2 +-
doc/src/sgml/config.sgml           |  3 +-
doc/src/sgml/indexam.sgml          |  3 +-
doc/src/sgml/indices.sgml          |  6 ++--
doc/src/sgml/monitoring.sgml       |  2 +-
doc/src/sgml/ref/create_table.sgml |  4 ++-
doc/src/sgml/storage.sgml          | 70 ++++++++++++++++++++++++++++++++++++++
9 files changed, 86 insertions(+), 11 deletions(-)