Add and fix documentation

- Add more documentation around utils functions
- Fix documentation about default formatting plugins
- Add extra documentation of filenames_from predicate parameter
- Add test for the default parameter of flake8.utils.fnmatch

tests/unit/test_utils.py

@@ -64,6 +64,12 @@ def test_fnmatch(filename, patterns, expected):
     assert utils.fnmatch(filename, patterns) is expected
 
 
+def test_fnmatch_returns_the_default_with_empty_default():
+    """The default parameter should be returned when no patterns are given."""
+    sentinel = object()
+    assert utils.fnmatch('file.py', [], default=sentinel) is sentinel
+
+
 def test_filenames_from_a_directory():
     """Verify that filenames_from walks a directory."""
     filenames = list(utils.filenames_from('flake8/'))