dm-crypt: Document integrity_key_size option.

This patch adds documentation for new option introduced in commit
4441686b24a1 ("dm-crypt: Allow to specify the integrity key size as option").

Signed-off-by: Milan Broz <[e-mail]>
Signed-off-by: Mikulas Patocka <[e-mail]>

Documentation/admin-guide/device-mapper/dm-crypt.rst

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     integrity for the encrypted device. The additional space is then
     used for storing authentication tag (and persistent IV if needed).
 
+integrity_key_size:<bytes>
+    Optionally set the integrity key size if it differs from the digest size.
+    It allows the use of wrapped key algorithms where the key size is
+    independent of the cryptographic key size.
+
 sector_size:<bytes>
     Use <bytes> as the encryption unit instead of 512 bytes sectors.
     This option can be in range 512 - 4096 bytes and must be power of two.