pgsql: Add notes about updating disk and shared memory size information

Log Message:
-----------
Add notes about updating disk and shared memory size information in the
documentation when doing new major release.

Modified Files:
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pgsql/src/tools:
RELEASE_CHANGES (r1.91 -> r1.92)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/tools/RELEASE_CHANGES?r1=1.91&r2=1.92)