Update the documentation in the TFP requirements.txt.

PiperOrigin-RevId: 572353174

requirements.txt

@@ -1,6 +1,11 @@
 # Requirements for Tensorflow Privacy.
 #
-# If you add a dependency here, also add the dependency to setup.py.
+# If you add a *new* dependency and it is required by the TensorFlow Federated
+# package, also add the dependency to `setup.py`.
+#
+# If you update the version of an *existing* dependency and it is required by
+# the TensorFlow Federated package, also update the version of the dependency in
+# `setup.py`.
 #
 # *   For packages that have a stable release, we use a version that is
 #     compatible with that release (e.g. `~=x.y`). See