doc: Fix explanation related to pg_shmem_allocations

Offsets are shown as NULL only for anonymous allocations.

Author: Benoit Lobréau
Reviewed-by: Kyotaro Horiguchi
Discussion: https://postgr.es/m/CAPE8EZ5Lnoyqoz7aZpvQM0E8sW+hw+k6G2NULe+m4arFRrA1aA@mail.gmail.com
Backpatch-through: 13

doc/src/sgml/catalogs.sgml

@@ -12493,7 +12493,7 @@ SELECT * FROM pg_locks pl LEFT JOIN pg_prepared_xacts ppx
       </para>
       <para>
        The offset at which the allocation starts. NULL for anonymous
-       allocations and unused memory.
+       allocations, since details related to them are not known.
       </para></entry>
      </row>