[Icons] explain svg colors and improve example configuration

Author: dbu

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
 ~~~~~~~~~
@@ -333,7 +336,7 @@ Now, all icons will have the ``fill`` attribute set to ``currentColor`` by defau
     # renders "user-profile.svg" with fill="currentColor"
     {{ ux_icon('user-profile') }}
 
-    # renders "user-profile.svg" with fill="red"
+    # renders "user-profile.svg" with fill="currentColor"
     {{ ux_icon('user-profile', {fill: 'red'}) }}
 
 Icon Aliases