pgsql: doc: Fix format, incorrect structure names and markup inconsiste

doc: Fix format, incorrect structure names and markup inconsistencies

Author: Alexander Lakhin
Discussion: /messages/by-id/a2345841-10a5-4eef-257c-02302347cf39@gmail.com
Backpatch-through: 13

Branch
------
REL_13_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/610394c7b876bc841464aad38f8abed09d63924e

Modified Files
--------------
doc/src/sgml/datetime.sgml | 8 ++++----
doc/src/sgml/func.sgml | 2 +-
doc/src/sgml/libpq.sgml | 29 ++++++++++++++++-------------
doc/src/sgml/monitoring.sgml | 8 ++++----
doc/src/sgml/protocol.sgml | 5 +++--
5 files changed, 28 insertions(+), 24 deletions(-)