CCBC-512: Reword `lock` option in lcb_CMDGET

The key is accessible via get and other routines, however it cannot be
modified via the mutation operations, and cannot be locked again during
the duration of the lock.

Change-Id: Ib0e1c437f9f80d7fa7b23ab7f89c52082e4c3f12
Reviewed-on: http://review.couchbase.org/41380
Reviewed-by: Matt Ingenthron <matt@couchbase.com>
Tested-by: Mark Nunberg <mnunberg@haskalah.org>