Clarify what "requested_time" should be set to when "activation":"mode" is set to "activate_immediate" (#175)

It's unclear what "requested_time" should be set to when "activation":"mode" is set to "activate_immediate".  Should it be null or absent or either.  The examples show it set to null but absent doesn't seem unreasonable so I've gone with that option. Please amend as required.

Author: Pete-Cordell

APIs/schemas/activation-schema.json

@@ -22,7 +22,7 @@
       }]
     },
     "requested_time": {
-      "description": "String formatted TAI timestamp (<seconds>:<nanoseconds>) indicating time (absolute or relative) for activation. Should be null or not present if 'mode' is null.",
+      "description": "String formatted TAI timestamp (<seconds>:<nanoseconds>) indicating time (absolute or relative) for activation. Should be null or not present if 'mode' is null or activate_immediate.",
       "anyOf": [{
         "type": "string",
         "pattern": "^[0-9]+:[0-9]+$"