MB-10095: Add cbepctl description for tap_throttle_cap_pcnt

Change-Id: I7dfc345a8808e7764f256975c0a7d8307675d66a
Reviewed-on: http://review.couchbase.org/33471
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>