docs: add documentation for EsatimatorTrial callbacks

docs/reference/api/estimator.txt

@@ -47,6 +47,40 @@ API or Native API.
     :members: cache_train_dataset, cache_validation_dataset
     :member-order: bysource
 
+
+Callbacks
+~~~~~~~~~
+
+To execute arbitrary Python functionality during the lifecycle of a
+``EstimatorTrial``, ``determined.estimator.RunHook`` extends
+`tf.estimator.SessionRunHook <https://www.tensorflow.org/api_docs/python/tf/estimator/SessionRunHook/>`_.
+
+.. autoclass:: determined.estimator.RunHook
+    :members: on_checkpoint_load, on_checkpoint_end
+
+
+Example usage of ``determined.estimator.RunHook`` which adds custom metadata checkpoints:
+
+.. code:: python
+
+    class MyHook(determined.estimator.RunHook):
+        def __init__(self, context, metadata) -> None:
+            self._context = context
+            self._metadata = metadata
+
+        def on_checkpoint_end(self, checkpoint_path) -> None:
+            with open(str(checkpoint_dir.joinpath("metadata.txt")), "w") as fp:
+                fp.write(self._metadata)
+
+    class MyEstimatorTrial(determined.estimator.EstimatorTrial) -> None:
+        ...
+
+        def build_train_spec(self) -> tf.estimator.TrainSpec:
+            return tf.estimator.TrainSpec(
+                make_input_fn(),
+                hooks=[MyHook(self.context, "my_metadata")],
+            )
+
 Examples
 --------