pgsql: Doc: clarify use of NULL to drop comments and security labels.

Doc: clarify use of NULL to drop comments and security labels.

This was only mentioned in the description of the text/label, which
are marked as being in quotes in the synopsis, which can cause
confusion (as witnessed on IRC).

Also separate the literal and NULL cases in the parameter list, per
suggestion from Tom Lane.

Also add an example of dropping a security label.

Dagfinn Ilmari Mannsåker, with some tweaks by me

Discussion: https://postgr.es/m/87sffqk4zp.fsf@wibble.ilmari.org

Branch
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REL_12_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/dbe294644380e6facd63aeaf3975af6188a219b9

Modified Files
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doc/src/sgml/ref/comment.sgml | 16 ++++++++++++----
doc/src/sgml/ref/security_label.sgml | 29 ++++++++++++++++++++++-------
2 files changed, 34 insertions(+), 11 deletions(-)