message: Add more explicit carve-out for experimental proto fields.
diff: @@ -67,7 +67,9 @@ backward incompatible ways between minor releases. These include:
  * **Experimental APIs**: The @{tf.contrib} module and its submodules in Python
  and any functions in the C API or fields in protocol buffers that are
  - explicitly commented as being experimental.
  + explicitly commented as being experimental. In particular, any field in a
  + protocol buffer which is called "experimental" and all its fields and
  + submessages can change at any time.
  * **Other languages**: TensorFlow APIs in languages other than Python and C,
  such as:
  