pgsql: Doc: add comment about bug fixed in back branches as of 3f7323cb

Doc: add comment about bug fixed in back branches as of 3f7323cbb.

While the bug I just fixed in the back branches doesn't exist in
HEAD, the requirement that MULTIEXPR SubPlans not share output
parameters still does.  Add a comment to memorialize that, because
perhaps it could be an issue again someday.

Discussion: https://postgr.es/m/17596-c5357f61427a81dc@postgresql.org

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/d1ce745db2d7db193c0be461db3cd34edd776dac

Modified Files
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src/backend/executor/nodeSubplan.c | 15 +++++++++++++++
1 file changed, 15 insertions(+)