dm integrity: document allow_discard option

Add decription of the allow_discard option added in commit
84597a44.
Signed-off-by: Milan Broz <gmazyland@gmail.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>

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

@@ -182,12 +182,15 @@ fix_padding
 	space-efficient. If this option is not present, large padding is
 	used - that is for compatibility with older kernels.
 
-
-The journal mode (D/J), buffer_sectors, journal_watermark, commit_time can
-be changed when reloading the target (load an inactive table and swap the
-tables with suspend and resume). The other arguments should not be changed
-when reloading the target because the layout of disk data depend on them
-and the reloaded target would be non-functional.
+allow_discards
+	Allow block discard requests (a.k.a. TRIM) for the integrity device.
+	Discards are only allowed to devices using internal hash.
+
+The journal mode (D/J), buffer_sectors, journal_watermark, commit_time and
+allow_discards can be changed when reloading the target (load an inactive
+table and swap the tables with suspend and resume). The other arguments
+should not be changed when reloading the target because the layout of disk
+data depend on them and the reloaded target would be non-functional.
 
 
 The layout of the formatted block device: