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
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master

Details
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https://git.postgresql.org/pg/commitdiff/b2a76bb7d0550fbfdc028e5bda4853d05444398a

Modified Files
--------------
doc/src/sgml/mvcc.sgml | 7 ++++---
doc/src/sgml/ref/comment.sgml | 5 +++++
2 files changed, 9 insertions(+), 3 deletions(-)