pgsql: In pg_upgrade, add C comment about why we don't try to do shared

In pg_upgrade, add C comment about why we don't try to do shared file
writes for logging.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/43aa40e1551b9e8d30b376de6d6a9b976ae54332

Modified Files
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contrib/pg_upgrade/pg_upgrade.h | 4 ++++
1 files changed, 4 insertions(+), 0 deletions(-)