Move module-level comment out of _RNNCell class (#8458)

* Move module-level comment out of _RNNCell class

The _RNNCell class docstring contained a description of the module
itself. I moved it back to the module's docstring.
I also fixed a comment that wasn't correctly indented.

* Reorganizing rnn_cell_impl module docstrings

- Moves the module docstring of `python.ops.rnn_cell_impl` to
`contrib.rnn.python.ops.core_rnn_cell_impl`, where the actual
implementation of all the LSTM cells is located.
- Adds a module docstring to `python.ops.rnn_cell_impl`, explaining
that the module contains an abstract definition of the `RNNCell` and
that the actual implementations are located in `contrib`.