Add note about PackageMetadata protocol in the docs.

Author: saaketp

docs/using.rst

@@ -162,6 +162,12 @@ all the metadata in a JSON-compatible form per PEP 566::
     >>> wheel_metadata.json['requires_python']
     '>=2.7, !=3.0.*, !=3.1.*, !=3.2.*, !=3.3.*'
 
+.. note::
+
+    The actual type of the object returned by ``metadata()`` is an
+    implementation detail, it should be accessed only through the interface
+    described by the ``PackageMetadata`` protocol documented here and at
+    https://importlib-metadata.readthedocs.io/en/latest/api.html#importlib_metadata.PackageMetadata
 
 .. _version: