doc: add comment about callOptions executor behavior (#8913)

Author: YifeiZhuang

api/src/main/java/io/grpc/CallOptions.java

@@ -358,6 +358,11 @@ public <T> T getOption(Key<T> key) {
     return key.defaultValue;
   }
 
+  /**
+   * Returns the executor override to use for this specific call, or {@code null} if there is no
+   * override. The executor is only for servicing this one call, so is not safe to use after
+   * {@link ClientCall.Listener#onClose}.
+   */
   @Nullable
   public Executor getExecutor() {
     return executor;