Expanded font outline documentation to better explain effects of values.

Author: cherez

docs/reST/ref/font.rst

@@ -316,7 +316,8 @@ solves no longer exists, it will likely be removed in the future.
 
       When set to 0, the font will be drawn normally. When positive,
       the text will be drawn as a hollow outline. This can be drawn
-      underneath unoutlined text to create a text outline effect.
+      underneath unoutlined text to create a text outline effect. Larger values
+      produce a thicker outline. Negative values are not valid.
 
       .. versionadded:: 2.5.6