pgsql: Small improvements in comments in async.c.

Small improvements in comments in async.c.

We seem to have lost a line somewhere along the way in the comment block
that discusses async.c's locks, because it suddenly refers to "both locks"
without previously having mentioned more than one.  Add a sentence to make
that read more sanely.  Also, refer to the "pos of the slowest backend"
not the "tail of the slowest backend", since we have no per-backend value
called "tail".

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/6057f61b4d511d8321f295d827ced13809e5a1ae

Modified Files
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src/backend/commands/async.c |    3 ++-
1 file changed, 2 insertions(+), 1 deletion(-)