Make constants constant, fix broken ref in docs

Use tuples instead of lists in module `fauxfactory.constants`. The built-in
tests function correctly:

    $ make test
    python -m unittest discover --start-directory tests --top-level-directory .
    .................................................................................................................................................................
    ----------------------------------------------------------------------
    Ran 161 tests in 0.934s

    OK

Make some changes to documentation:

* Drop some comments from `fauxfactory.constants`. An explanation of lorem ipsum
  seems unnecessary, for example.
* Move remaining descriptions in to the module-wide docstring. This allows
  descriptions to be found both by Sphinx and the `help` built-in. Also include
  the `fauxfactory.constants` module in the API documentation. These changes
  allow documentation to be built with zero warnings.
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