Expose useful tf.util.* methods to our docs (#1580)
Expose useful tf.util.* methods to our docs (#1580)

Fixes https://github.com/tensorflow/tfjs/issues/1206

Expose and document:
- `tf.util.shuffle`
- `tf.util.assert`
- `tf.util.flatten`
- `tf.util.sizeFromShape`
- `tf.util.now`

tensorflow/tfjs,tensorflow/tfjs-core,tensorflow/tfjs,tensorflow/tfjs,tensorflow/tfjs,tensorflow/tfjs,tensorflow/tfjs-core,tensorflow/tfjs-core,tensorflow/tfjs,tensorflow/tfjs-core,tensorflow/tfjs-core,tensorflow/tfjs,tensorflow/tfjs,tensorflow/tfjs,tensorflow/tfjs-core