pgsql: Add comment for "is_internal" parameter

Add comment for "is_internal" parameter

This was missed in my commit f4c4335 of 9.3 vintage, so backpatch to
that.

Branch
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REL9_4_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/2570e28827e3ac4a316300f92dfaf448e55c8e0c

Modified Files
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src/backend/catalog/heap.c |    1 +
1 file changed, 1 insertion(+)