Overwrite single HDF5 dataset without overwriting file

Adds ability to overwrite a single table within an HDF5 file without having to overwrite the entire file (and other included datasets) by using a combination of `overwrite=True` and `append=True` arguments to the `Table.write` method. See #2624.

Includes a test for this combination of arguments, as well as a mention of this functionality in the `docs/io/unified.rst` doc page.

Also includes two small commits to improve the `io.misc.hdf5.py` module docstrings and PEP8 compliance..