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_12_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/7b96f525cdec1b59c229457099653de254f9ddbf

Modified Files
--------------
doc/src/sgml/catalogs.sgml              |   7 +-
doc/src/sgml/config.sgml                |   6 +-
doc/src/sgml/filelist.sgml              |   1 +
doc/src/sgml/monitoring.sgml            |   3 +-
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/wal.sgml                   |   6 +-
doc/src/sgml/xact.sgml                  | 200 ++++++++++++++++++++++++++++++++
11 files changed, 266 insertions(+), 36 deletions(-)