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

Branch
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REL_18_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/c33e55ac91d28235932bc4e38604f13360d9b731

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