Update CHANGELOG.md

Author: daymxn

firebase-vertexai/CHANGELOG.md

@@ -1,5 +1,7 @@
 # Unreleased
-* [fixed] Added support for `HarmBlockThreshold.OFF`.
+* [feature] Added support for `HarmBlockThreshold.OFF`. See the
+  [model documentation](https://cloud.google.com/vertex-ai/generative-ai/docs/multimodal/configure-safety-filters#how_to_configure_content_filters){: .external}
+  for more information.
 
 # 16.3.0
 * [feature] Emits a warning when attempting to use an incompatible model with