pgsql: Document the recently-understood hazard that a rollback can

Log Message:
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Document the recently-understood hazard that a rollback can release row-level
locks that logically should not be released, because when a subtransaction
overwrites XMAX all knowledge of the previous lock state is lost.  It seems
unlikely that we will be able to fix this before 8.3...

Tags:
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REL8_1_STABLE

Modified Files:
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    pgsql/doc/src/sgml/ref:
        select.sgml (r1.90 -> r1.90.2.1)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/ref/select.sgml.diff?r1=1.90&r2=1.90.2.1)