Fix colors.completion.fg default and description.

Update the description to mention the number of columns and change the
default to ["white", "white", "white"] to make it more obvious that
multiple colors can be specified. This also satisfies the config test
that expects the default value for ListOrValue types to be a list.

One other test had to be tweaked to use a config option that is still
just a QtColor rather than a ListOrValue.

While it is possible to provide just two colors, it is "undefined
behavior". It will use the first color as the third color, but that is
an artifact of the implementation and therefore not documented (though
also not an error, as it is harmless).

qutebrowser/config/configdata.yml

@@ -1540,15 +1540,15 @@ zoom.text_only:
 ## colors
 
 colors.completion.fg:
-  default: white
+  default: ["white", "white", "white"]
   type:
     name: ListOrValue
     valtype: QtColor
   desc: >-
     Text color of the completion widget.
 
-    May be a single value to specify the color for all columns, or a list
+    May be a single color to use for all columns or a list of three colors,
-    specifying a different color for each column.
+    one for each column.
 
 colors.completion.odd.bg:
   default: '#444444'

tests/unit/config/test_config.py

@@ -400,7 +400,7 @@ class TestConfig:
         assert not conf._mutables
 
     def test_get_obj_simple(self, conf):
-        assert conf.get_obj('colors.completion.fg') == 'white'
+        assert conf.get_obj('colors.completion.category.fg') == 'white'
 
     @pytest.mark.parametrize('option', ['content.headers.custom',
                                         'keyhint.blacklist',