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: https://postgr.es/m/_0HDHIGcCdCsUyXn22QwI2FEuNR6Fs71rtgGX6hfyBlUh5rrnE2qMmvIFu9EY4Pijr2gUmJEAXCjuNU2Oxku9TryLp9CdHllpsCfN3gD0-Y=@pm.me
Backpatch-through: 10

Author: michaelpq

doc/src/sgml/mvcc.sgml

@@ -950,10 +950,11 @@ ERROR:  could not serialize access due to read/write dependencies among transact
         <para>
          Acquired by <command>VACUUM</command> (without <option>FULL</option>),
          <command>ANALYZE</command>, <command>CREATE INDEX CONCURRENTLY</command>,
+         <command>CREATE STATISTICS</command>, <command>COMMENT ON</command>,
          <command>REINDEX CONCURRENTLY</command>,
-         <command>CREATE STATISTICS</command>, and certain <link linkend="sql-alterindex"><command>ALTER
-         INDEX</command></link> and <link linkend="sql-altertable"><command>ALTER TABLE</command></link> variants (for full
-         details see the documentation of these commands).
+         and certain <link linkend="sql-alterindex"><command>ALTER INDEX</command></link>
+         and <link linkend="sql-altertable"><command>ALTER TABLE</command></link> variants
+         (for full details see the documentation of these commands).
         </para>
        </listitem>
       </varlistentry>

doc/src/sgml/ref/comment.sgml

@@ -90,6 +90,11 @@ COMMENT ON
    Comments are automatically dropped when their object is dropped.
   </para>
 
+  <para>
+   A <literal>SHARE UPDATE EXCLUSIVE</literal> lock is acquired on the
+   object to be commented.
+  </para>
+
   <para>
    For most kinds of object, only the object's owner can set the comment.
    Roles don't have owners, so the rule for <literal>COMMENT ON ROLE</literal> is