s390/s390dbf.txt: Add doc: Debug views are removed in debug_unregister()

Signed-off-by: NMichael Holzheu <holzheu@linux.vnet.ibm.com>
Signed-off-by: NMartin Schwidefsky <schwidefsky@de.ibm.com>

Documentation/s390/s390dbf.txt

@@ -143,7 +143,8 @@ Parameter:     id:   handle for debug log
 
 Return Value:  none 
 
-Description:   frees memory for a debug log     
+Description:   frees memory for a debug log and removes all registered debug
+	       views.
                Must not be called within an interrupt handler 
 
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