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_13_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/4df5462f345b5e50cedf42300cad8616c7df3f6d

Modified Files
--------------
doc/src/sgml/catalogs.sgml              |   7 +-
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/wal.sgml                   |   6 +-
doc/src/sgml/xact.sgml                  | 205 ++++++++++++++++++++++++++++++++
14 files changed, 292 insertions(+), 44 deletions(-)