pypi: switch to standard file_name, add links docs

- Use the standard `file_name` qualifier, not `filename`.
- Link to the Python Packaging User Guide for filename conventions.

Thanks to @pombredanne for the suggestions.

Signed-off-by: Mark Lodato <[email protected]>

PURL-TYPES.rst

@@ -519,10 +519,15 @@ pypi
 - PyPI treats ``-`` and ``_`` as the same character and is not case sensitive.
   Therefore a PyPI package ``name`` must be lowercased and underscore ``_``
   replaced with a dash ``-``.
-- The ``filename`` qualifier selects a particular distribution file (case-sensitive).
+- The ``file_name`` qualifier selects a particular distribution file
+  (case-sensitive). For naming convention, see the Python Packaging User Guide on
+  `source distributions <https://packaging.python.org/en/latest/specifications/source-distribution-format/#source-distribution-file-name>`_
+  `binary distributions <https://packaging.python.org/en/latest/specifications/binary-distribution-format/#file-name-convention>`_,
+  and `platform compatibility tags <https://packaging.python.org/en/latest/specifications/platform-compatibility-tags/>`_.
 - Examples::
 
       pkg:pypi/[email protected]
+      pkg:pypi/[email protected]?filename=Django-1.11.1.tar.gz
       pkg:pypi/[email protected]?filename=Django-1.11.1-py2.py3-none-any.whl
       pkg:pypi/[email protected]