pgsql: Update comments related to the crash-safety of the visibility ma

Update comments related to the crash-safety of the visibility map.

In hio.c, document how we avoid deadlock with respect to visibility map
buffer locks.  In visibilitymap.c, update the LOCKING section of the
file header comment.

Both oversights noted by Heikki Linnakangas.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/f70648d5a16b4b048cacd04b82079e6734449bc2

Modified Files
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src/backend/access/heap/hio.c           |    4 ++++
src/backend/access/heap/visibilitymap.c |   16 +++++++++++++---
2 files changed, 17 insertions(+), 3 deletions(-)