Clarify start_time documentation

Signed-off-by: Stefan Brus <[email protected]>

Author: stefan-brus-frequenz

proto/frequenz/api/dispatch/dispatch.proto

@@ -156,6 +156,9 @@ message DispatchCreateRequest {
   string type = 2;
 
   // The start time
+  // When creating a dispatch, ensure that the starting timestamp is set to
+  // the current time or any future time.
+  // Timestamps earlier than the current time are not allowed.
   google.protobuf.Timestamp start_time = 3;
 
   // The end time
@@ -186,6 +189,9 @@ message DispatchUpdateRequest {
   optional string type = 3;
 
   // The start time
+  // When updating a dispatch, ensure that the starting timestamp is set to
+  // the current time or any future time.
+  // Timestamps earlier than the current time are not allowed.
   google.protobuf.Timestamp start_time = 4;
 
   // The end time