Correct the watcher state files in the man page

The move to per-group state files forgot to update the man page of
ganeti-watcher; let's fix this and add some more details on what
removal means and when it's needed.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: René Nussbaumer <rn@google.com>

man/ganeti-watcher.rst

@@ -68,12 +68,24 @@ execute much later than the watcher submits them.
 FILES
 -----
 
-The command has a state file located at
-``@LOCALSTATEDIR@/lib/ganeti/watcher.data`` (only used on the master)
-and a log file at ``@LOCALSTATEDIR@/log/ganeti/watcher.log``. Removal
-of either file will not affect correct operation; the removal of the
-state file will just cause the restart counters for the instances to
-reset to zero.
+The command has a set of state files (one per group) located at
+``@LOCALSTATEDIR@/lib/ganeti/watcher.GROUP-UUID.data`` (only used on the
+master) and a log file at
+``@LOCALSTATEDIR@/log/ganeti/watcher.log``. Removal of either file(s)
+will not affect correct operation; the removal of the state file will
+just cause the restart counters for the instances to reset to zero, and
+mark nodes as freshly rebooted (so for example DRBD minors will be
+re-activated).
+
+In some cases, it's even desirable to reset the watcher state, for
+example after maintenance actions, or when you want to simulate the
+reboot of all nodes, so in this case, you can remove all state files::
+
+    rm -f @LOCALSTATEDIR@/lib/ganeti/watcher.*.data
+    rm -f @LOCALSTATEDIR@/lib/ganeti/watcher.*.instance-status
+    rm -f @LOCALSTATEDIR@/lib/ganeti/instance-status
+
+And then re-run the watcher.
 
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