Expand setup.py test/py.test options description

- Use sphinx option markup for clarity/cross-referencing
- Put -h/--help at the top of the option list
- Add description for --markers option of setup.py test
- Add description for --repeat option of py.test

/cc @ximenesuk
It may well pre-date this PR but it's a bit odd having a link to the option within the option. It makes some sense when the link is to elsewhere on the page (cf --markers). It does though render the option heading with a yellow background! Maybe "This option..." would be better? (ditto -m)