message: Add docs for tf.strings.lower
diff: op {
  graph_op_name: "StringLower"
  - description: "Converts each string in the input Tensor to lowercase."
  + summary: "Converts all uppercase characters into their respective lowercase replacements."
  + description: <<END
  +Example:
  +```
  +>>> tf.strings.lower("CamelCase string and ALL CAPS")
  +<tf.Tensor: shape=(), dtype=string, numpy=b'camelcase string and all caps'>
  +```
  +END
  }
  