pgsql: Rewrite comment for shm_mq_receive_bytes.

Rewrite comment for shm_mq_receive_bytes.

The comment and the code diverged at some point before the initial
commit of this feature, and I failed to notice.

Noted by Tom Lane.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/250f8a7bbe83a7dbc9618c0a506f7170ab610e4b

Modified Files
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src/backend/storage/ipc/shm_mq.c |   11 ++++++-----
1 file changed, 6 insertions(+), 5 deletions(-)