[gtkmm] Gdk::RGBA: Update ctor(ustring) docs from GTK+

[Daniel Boles <dboles src gnome org>]

commit 883e8dcb1429b20bbc761f934e2614b47af1d65b
Author: Daniel Boles <dboles src gnome org>
Date:   Thu Jul 6 13:53:04 2017 +0100

    Gdk::RGBA: Update ctor(ustring) docs from GTK+
    
    https://bugzilla.gnome.org/show_bug.cgi?id=784483

 gdk/src/rgba.hg |   23 ++++++++++++++++++++---
 1 files changed, 20 insertions(+), 3 deletions(-)
---
diff --git a/gdk/src/rgba.hg b/gdk/src/rgba.hg
index 8a3a049..b236734 100644
--- a/gdk/src/rgba.hg
+++ b/gdk/src/rgba.hg
@@ -43,9 +43,26 @@ public:
    */
   RGBA();
 
-  /** Instantiate a new Gdk::RGBA.
-   * The text string can be in any of the forms accepted by XParseRGBA; these include names for a color from 
rgb.txt,
-   * such as DarkSlateGray, or a hex specification such as 305050.
+  /** Parses a textual representation of a color and creates a new Gdk::RGBA
+   * with the corresponding values in its <tt>red</tt>, <tt>green</tt>,
+   * <tt>blue</tt> and <tt>alpha</tt> fields.
+   *
+   * The string can be one of:
+   *
+   * <ul>
+   *   <li>A standard name (taken from the X11 rgb.txt file)</li>
+   *   <li>A hexadecimal value in the form <tt>#rgb</tt>, <tt>#rrggbb</tt>,
+   *     <tt>#rrrgggbbb</tt> or </tt>#rrrrggggbbbb</tt></li>
+   *   <li>An RGB color in the form <tt>rgb(r,g,b)</tt> (In this case the color will have full opacity)</li>
+   *   <li>An RGBA color in the form <tt>rgba(r,g,b,a)</tt></li>
+   * </ul>
+   *
+   * Where <tt>r</tt>, <tt>g</tt>, <tt>b</tt> and <tt>a</tt> are respectively
+   * the red, green, blue and alpha color values. In the last two cases,
+   * <tt>r</tt> <tt>g</tt> and <tt>b</tt> are either integers in the range 0 to
+   * 255 or percentage values in the range 0% to 100%, and <tt>a</tt> is a
+   * floating point value in the range 0 to 1.
+   *
    * @param value the string specifying the color.
    */
   explicit RGBA(const Glib::ustring& value);