Updated Dart_Post docstring to mention Dart_PostCObject.

fixes https://github.com/dart-lang/sdk/issues/49524

TEST=n/a, documentation

Change-Id: Ia5154e9c2c1d6f6cce46e9bceb49f315cb840bc3
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/252681
Reviewed-by: Ryan Macnak <rmacnak@google.com>
Commit-Queue: Ryan Macnak <rmacnak@google.com>
diff --git a/runtime/include/dart_api.h b/runtime/include/dart_api.h
index f45925e..bbc1291 100644
--- a/runtime/include/dart_api.h
+++ b/runtime/include/dart_api.h
@@ -1645,6 +1645,8 @@
  *
  * Requires there to be a current isolate.
  *
+ * For posting messages outside of an isolate see \ref Dart_PostCObject.
+ *
  * \param port_id The destination port.
  * \param object An object from the current isolate.
  *