DOC: Clarified PyTables "natural" names
The HDF5 documentation "Notes & Caveats" defines a PyTables NaturalNameWarning vaguely, and incorrectly states that a natural name may not begin with an underscore. I clarified the definition of a natural identifier. The relevant regex is defined [here](https://github.com/PyTables/PyTables/blob/2fde39957c7b5263dd4d0b11aa797900250f16bc/tables/path.py#L39) in the PyTables source.

thanks!
