[Icons] explain svg colors and improve example configuration

Author: dbu

Diff for: src/Icons/doc/index.rst

@@ -83,6 +83,9 @@ define the HTML attributes added to the ``<svg>`` element:
     {{ ux_icon('user-profile', {height: '16px', width: '16px', 'aria-hidden': true}) }}
     {# renders <svg height="16" width="16" aria-hidden="true"> ... </svg> #}
 
+Typically, icon SVG use `fill='currentColor'` to inherit the color of the containing
+element. You can use CSS to set the color either on the containing element or in a class
+or style attribute of the icon itself.
 
 Icon Sets
 ~~~~~~~~~