dm cache: document metadata device is exclussive to a cache

A cache's metadata device may not be shared by multiple cache devices.
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Signed-off-by: Alasdair G Kergon <agk@redhat.com>
Acked-by: Joe Thornber <ejt@redhat.com>

Documentation/device-mapper/cache.txt

@@ -50,7 +50,8 @@ other parameters detailed later):
    which are dirty, and extra hints for use by the policy object.
    This information could be put on the cache device, but having it
    separate allows the volume manager to configure it differently,
-   e.g. as a mirror for extra robustness.
+   e.g. as a mirror for extra robustness.  This metadata device may only
+   be used by a single cache device.
 
 Fixed block size
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