api3: reference lcb_sched_*, not mcreq_sched_* in docs

The former are public API wrappers for the latter

Change-Id: I0229650e182a704f8164f94842f7007396426598
Reviewed-on: http://review.couchbase.org/38088
Tested-by: Mark Nunberg <mnunberg@haskalah.org>
Reviewed-by: Brett Lawson <brett19@gmail.com>