pgsql: doc: Mention the level of locks taken on objects in COMMENT

doc: Mention the level of locks taken on objects in COMMENT

This information was nowhere to be found. This adds one note on the
page of COMMENT, and one note in the section dedicated to explicit
locking, both telling that a SHARE UPDATE EXCLUSIVE lock is taken on the
object commented.

Author: Nikolai Berkoff
Reviewed-by: Laurenz Albe
Discussion: /messages/by-id/_0HDHIGcCdCsUyXn22QwI2FEuNR6Fs71rtgGX6hfyBlUh5rrnE2qMmvIFu9EY4Pijr2gUmJEAXCjuNU2Oxku9TryLp9CdHllpsCfN3gD0-Y=@pm.me
Backpatch-through: 10

Branch
------
REL_13_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/3ca40a2b18a0638994e35160dfc8d392b1aef898

Modified Files
--------------
doc/src/sgml/mvcc.sgml | 9 +++++----
doc/src/sgml/ref/comment.sgml | 5 +++++
2 files changed, 10 insertions(+), 4 deletions(-)