message: Add more description for a common use case of SequenceExample.
diff: @@ -199,7 +199,13 @@ message Example {
  // to determine if all features within the FeatureList must
  // have the same size. The same holds for this FeatureList across multiple
  // examples.
  -//
  +// - For sequence modeling, e.g.:
  +// http://colah.github.io/posts/2015-08-Understanding-LSTMs/
  +// https://github.com/tensorflow/nmt
  +// the feature lists represent a sequence of frames.
  +// In this scenario, all FeatureLists in a SequenceExample have the same
  +// number of Feature messages, so that the ith element in each FeatureList
  +// is part of the ith frame (or time step).
  // Examples of conformant and non-conformant examples' FeatureLists:
  //
  // Conformant FeatureLists:
  