Adds a comment for why we added an upper bound for protobuf dependency.

tensorboard/pip_package/requirements.txt

@@ -26,6 +26,8 @@ numpy >= 1.12.0
 # At the same time, any constraints we specify here must allow at least some
 # version to be installed that is also compatible with TensorFlow's constraints:
 # https://github.com/tensorflow/tensorflow/blob/9d22f4a0a9499c8e10a4312503e63e0da35ccd94/tensorflow/tools/pip_package/setup.py#L100-L107
+# We needed to add an upper bound restriction due to issue reported in:
+# https://github.com/protocolbuffers/protobuf/issues/13485
 protobuf >= 3.19.6, < 4.24
 requests >= 2.21.0, < 3
 setuptools >= 41.0.0  # Note: provides pkg_resources as well as setuptools