Added explanation of *.system values to settings page.

qutebrowser/config/configdata.py

@@ -103,6 +103,10 @@ SECTION_DESC = {
         " * A gradient as explained in http://qt-project.org/doc/qt-4.8/"
         "stylesheet-reference.html#list-of-property-types[the Qt "
         "documentation] under ``Gradient''.\n\n"
+        "A *.system value determines the color system to use for color "
+        "interpolation between similarly-named *.start and *.stop entries, "
+        "regardless of how they are defined in the options. "
+        "Valid values are 'rgb', 'hsv', and 'hsl'.\n\n"
         "The `hints.*` values are a special case as they're real CSS "
         "colors, not Qt-CSS colors. There, for a gradient, you need to use "
         "`-webkit-gradient`, see https://www.webkit.org/blog/175/introducing-"