keyusage: add documentation

doc/configuration.rst

@@ -438,3 +438,15 @@ of the limitations will be hopefully removed in the near future.
   - Legacy key import requires a private key.
   - Legacy key export is not implemented.
   - DS record export is not implemented.
+
+.. _dnssec-keyusage:
+
+DNSSEC keys used by multiple zones 
+----------------------------------
+
+Using same key for multiple zones with automatic key management is possible. 
+However, all zones must be listed in keyusage (keys directory) or they will be deleted,
+when they retire in any zone.
+
+If keys are added manually as published, but not active (for next rollover event), they are added automatically.
+