dm verity: Add documentation for try_verify_in_tasklet option

Add documentation that was missing from commit 5721d4e5 ("dm
verity: Add optional "try_verify_in_tasklet" feature").
Signed-off-by: Milan Broz <gmazyland@gmail.com>
Signed-off-by: Mike Snitzer <snitzer@kernel.org>

Documentation/admin-guide/device-mapper/verity.rst

@@ -141,6 +141,10 @@ root_hash_sig_key_desc <key_description>
     also gain new certificates at run time if they are signed by a certificate
     already in the secondary trusted keyring.
 
+try_verify_in_tasklet
+    If verity hashes are in cache, verify data blocks in kernel tasklet instead
+    of workqueue. This option can reduce IO latency.
+
 Theory of operation
 ===================