Documentation bug for Fits tables

The 'creating a new table file' sphinx documentation for astropy.io.fits seems to have some errors related to appending a primary HDU.

First, the description of appending a primary HDU:

      hdu = fits.PrimaryHDU(n)

uses an undefined variable (n).  Actually, it works in the doctest because n is defined in an earlier section unrelated to making tables, but it shouldn't be there.

Fixing that would be trivial, but the bigger issue is that - as far as I can tell - this command does not do what the docs say, which is to automatically append an empty primary HDU to the table HDU (a good thing, since you haven't specified any connection to the table).

Maybe what was intended was

     tbhdu.writeto(filename)

but I'm not sure enough of that to just submit a pull to fix it.