pgsql: Doc: clarify description of current-date/time functions.

Doc: clarify description of current-date/time functions.

Minor wordsmithing of the func.sgml paragraph describing
statement_timestamp() and allied functions: don't switch between
"statement" and "command" when those are being used to mean about
the same thing.

Also, add some text to protocol.sgml describing the perhaps-surprising
behavior these functions have in a multi-statement Query message.

Reported-by: P M <petermittere(at)gmail(dot)com>
Author: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Reviewed-by: Laurenz Albe <laurenz(dot)albe(at)cybertec(dot)at>
Reviewed-by: David G. Johnston <david(dot)g(dot)johnston(at)gmail(dot)com>
Discussion: https://postgr.es/m/175223006802.3157505.14764328206246105568@wrigleys.postgresql.org
Backpatch-through: 13

Branch
------
REL_13_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/0e2cc385b97ef53198006699b0322f155a213253

Modified Files
--------------
doc/src/sgml/func.sgml | 6 +++---
doc/src/sgml/protocol.sgml | 10 ++++++++++
2 files changed, 13 insertions(+), 3 deletions(-)