pgsql: Add C comment

Add C comment

Add C comment about why we throw an error if the pg_upgrade old/new
database counts don't match.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/034194470647b3de206fb42464d49a438853fa5e

Modified Files
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contrib/pg_upgrade/relfilenode.c | 6 ++++++
1 files changed, 6 insertions(+), 0 deletions(-)