Update force-upgrade action description (#133)

Author: carlcsaposs-canonical

actions.yaml

@@ -4,4 +4,11 @@
 resume-upgrade:
   description: Upgrade remaining units (after you manually verified that upgraded units are healthy).
 force-upgrade:
-  description: Force upgrade of this unit.
+  description: |
+    Potential of *data loss* and *downtime*
+        
+    Force upgrade of this unit.
+    
+    Use to
+    - force incompatible upgrade and/or
+    - continue upgrade if 1+ upgraded units have non-active status