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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master

Details
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https://git.postgresql.org/pg/commitdiff/66bc9d2d3e57acca20bcb0b6deb225abb01384bf

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(-)