pgsql: Adjust misleading comment in walsender.c.

Log Message:
-----------
Adjust misleading comment in walsender.c. We try to send all WAL data that's
been written out from shared memory, but the previous phrasing might be read
to say that we send only what's been fsync'd.

Modified Files:
--------------
pgsql/src/backend/replication:
walsender.c (r1.25 -> r1.26)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/replication/walsender.c?r1=1.25&r2=1.26)