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_11_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/80e591676bf374c632cfec1b61fde0ca468988f8

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_to.sgml       |   5 +-
doc/src/sgml/wal.sgml                   |   6 +-
doc/src/sgml/xact.sgml                  | 200 ++++++++++++++++++++++++++++++++
10 files changed, 262 insertions(+), 32 deletions(-)