pgsql: doc: add transaction processing chapter with internals info

doc:  add transaction processing chapter with internals info

This also adds references to this new chapter at relevant sections of
our documentation.  Previously much of these internal details were
exposed to users, but not explained.  This also updates RELEASE
SAVEPOINT.

Discussion: https://postgr.es/m/CANbhV-E_iy9fmrErxrCh8TZTyenpfo72Hf_XD2HLDppva4dUNA@mail.gmail.com

Author: Simon Riggs, Laurenz Albe

Reviewed-by: Bruce Momjian

Backpatch-through: 11

Branch
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REL_15_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/8592b56faf86be1a8684544ad3a93e2fe8f5aa1b

Modified Files
--------------
doc/src/sgml/config.sgml                |   6 +-
doc/src/sgml/datatype.sgml              |   3 +-
doc/src/sgml/filelist.sgml              |   1 +
doc/src/sgml/func.sgml                  |  20 +++-
doc/src/sgml/glossary.sgml              |   3 +-
doc/src/sgml/monitoring.sgml            |   6 +-
doc/src/sgml/pgrowlocks.sgml            |   3 +-
doc/src/sgml/postgres.sgml              |   1 +
doc/src/sgml/ref/release_savepoint.sgml |  62 ++++++----
doc/src/sgml/ref/rollback.sgml          |   8 +-
doc/src/sgml/ref/rollback_to.sgml       |   5 +-
doc/src/sgml/system-views.sgml          |   7 +-
doc/src/sgml/wal.sgml                   |   6 +-
doc/src/sgml/xact.sgml                  | 205 ++++++++++++++++++++++++++++++++
14 files changed, 292 insertions(+), 44 deletions(-)