Update gnmi-specification.md Target_Defined mode description

rpc/gnmi/gnmi-specification.md

@@ -1636,6 +1636,10 @@ with one of the following `modes`:
   driven (e.g., the changing of state of an entity based on an external
   trigger) then an `ON_CHANGE` subscription may be created, whereas if other
   data represents counter values, a `SAMPLE` subscription may be created.
+  The target is expected to choose and change a `sample_interval` dynamically.
+  If `sample_interval` is specified in the subscribe request, the target
+  `SHOULD` reject the subscription by closing the Subscribe RPC specifying an
+  `InvalidArgument (3)` error code.
 
 ##### 3.5.1.5.3 POLL Subscriptions