pgsql: Add C comment explaining why we can't exclude checking functions

Add C comment explaining why we can't exclude checking functions in the
pg_catalog schema, even though they are not explicitly dumped (they are
implicitly dumped, e.g. create language plperl).

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/20a3830b81d07aca1b8c3e954b4bc49d0847c51d

Modified Files
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contrib/pg_upgrade/function.c |    7 ++++++-
1 files changed, 6 insertions(+), 1 deletions(-)