dm-integrity: Document Inline mode for storing integrity data

This patch adds documentation for new 'I' mode for dm-integrity
introduced in commit
fb09876 ("dm-integrity: introduce the Inline mode").

Signed-off-by: Milan Broz <gmazyland@gmail.com>
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>

Author: mbroz

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

@@ -92,6 +92,11 @@ Target arguments:
 		allowed. This mode is useful for data recovery if the
 		device cannot be activated in any of the other standard
 		modes.
+	I - inline mode - in this mode, dm-integrity will store integrity
+		data directly in the underlying device sectors.
+		The underlying device must have an integrity profile that
+		allows storing user integrity data and provides enough
+		space for the selected integrity tag.
 
 5. the number of additional arguments