mbcache: document that "find" functions only return reusable entries

mb_cache_entry_find_first() and mb_cache_entry_find_next() only return
cache entries with the 'e_reusable' bit set.  This should be documented.

Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Reviewed-by: Jan Kara <jack@suse.cz>