Add FastxRecord to the docs

[nh13]

No description provided.

[jmarshall]

When we add a previously omitted class to api.rst, we suddenly find out whether its docstrings render well as reStructuredText or provide useful information at all. In this case, some of it's not great:

name
    unicode
        Type: name

These appear to be the default rendering for cdef public str from the .pxd file. (These are the only public str items in pysam.) “Unicode” is irrelevant as we're Python 3 only, and “Type” is just wrong.

Perhaps we can add some docstrings to the .pyx file to override this rendering and provide some meaningful information for these properties…