dm writecache: add "cleaner" and "max_age" to Documentation

Backfill missing Documentation.

Fixes: 93de44eb ("dm writecache: implement the "cleaner" policy")
Fixes: 3923d485 ("dm writecache: implement gradual cleanup")
Signed-off-by: Mike Snitzer <snitzer@redhat.com>

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

@@ -53,6 +53,21 @@ Constructor parameters:
 
 		- some underlying devices perform better with fua, some
 		  with nofua. The user should test it
+	cleaner
+		when this option is activated (either in the constructor
+		arguments or by a message), the cache will not promote
+		new writes (however, writes to already cached blocks are
+		promoted, to avoid data corruption due to misordered
+		writes) and it will gradually writeback any cached
+		data. The userspace can then monitor the cleaning
+		process with "dmsetup status". When the number of cached
+		blocks drops to zero, userspace can unload the
+		dm-writecache target and replace it with dm-linear or
+		other targets.
+	max_age n
+		specifies the maximum age of a block in milliseconds. If
+		a block is stored in the cache for too long, it will be
+		written to the underlying device and cleaned up.
 
 Status:
 1. error indicator - 0 if there was no error, otherwise error number
@@ -77,3 +92,5 @@ Messages:
 		5. resume the device, so that it will use the linear
 		   target
 		6. the cache device is now inactive and it can be deleted
+	cleaner
+		See above "cleaner" constructor documentation.