Fix first_execution_run_id comments on Cancel and Terminate RPCs

The comments on RequestCancelWorkflowExecution and
TerminateWorkflowExecution claimed that first_execution_run_id
validates against either the most recent or specified (by run id)
execution. In reality, the server unconditionally ignores the run id
when first_execution_run_id is set, always resolving to the most
recent execution for the workflow id.

The UpdateWorkflowExecution comment is left unchanged because its
server implementation does respect the run id when set.

Author: dandavison

temporal/api/workflowservice/v1/request_response.proto

@@ -737,9 +737,10 @@ message RequestCancelWorkflowExecutionRequest {
     string identity = 3;
     // Used to de-dupe cancellation requests
     string request_id = 4;
-    // If set, this call will error if the most recent (if no run id is set on
-    // `workflow_execution`), or specified (if it is) workflow execution is not part of the same
-    // execution chain as this id.
+    // If set, the run id on `workflow_execution` is ignored and the most recent
+    // workflow execution for the given workflow id is resolved. This call will
+    // then error if that execution is not part of the same execution chain as
+    // this id.
     string first_execution_run_id = 5;
     // Reason for requesting the cancellation
     string reason = 6;
@@ -887,9 +888,10 @@ message TerminateWorkflowExecutionRequest {
     temporal.api.common.v1.Payloads details = 4;
     // The identity of the worker/client
     string identity = 5;
-    // If set, this call will error if the most recent (if no run id is set on
-    // `workflow_execution`), or specified (if it is) workflow execution is not part of the same
-    // execution chain as this id.
+    // If set, the run id on `workflow_execution` is ignored and the most recent
+    // workflow execution for the given workflow id is resolved. This call will
+    // then error if that execution is not part of the same execution chain as
+    // this id.
     string first_execution_run_id = 6;
 
     // Links to be associated with the WorkflowExecutionTerminated event.