Improve docstring of data_kind() to include xarray grid

[seisman]

Description of proposed changes

Improve the docstrings of data_kind() and add one more doctest.

Reminders

• Run make format and make check to make sure the code follows the style guide.
• Add tests for new features or tests that would have caught the bug that you're fixing.
• Add new public functions/methods/classes to doc/api/index.rst.
• Write detailed docstrings for all functions/methods.
• If adding new functionality, add an example to docstrings or tutorials.

[weiji14]

This is good. Just one suggestion to change the title to something like: "Improve docstring of data_kind() to include xarray grid".

Also, this reminds me that I'll need to do the same for the 'buffer' data_kind docstring at #576.