mcreq: clean up flags definitions

This commit removes the unused F_PASSTHROUGH flag and documents the
F_DETACHED flag

Change-Id: I1b2d2a82def7eb94e6159d8743a65785fb71ba6d
Reviewed-on: http://review.couchbase.org/39821
Reviewed-by: Brett Lawson <brett19@gmail.com>
Tested-by: Mark Nunberg <mnunberg@haskalah.org>