[gimp-help] src: explain a bit more difference between color pickers.

[Jehan <jehanp src gnome org>]

commit 7b64b20b2a8ca8b64ae3aae54628e7d5d9c7ff83
Author: Jehan <jehan girinstud io>
Date:   Sat May 2 12:45:45 2020 +0200

    src: explain a bit more difference between color pickers.
    
    The color picker tool gets exactly what's stored in the image data,
    whereas the color picker from Colors dockable gets the color as
    displayed after full processing by system stack. The later is similar to
    what usual generic color picker software do.
    
    This explains possible (usually slight) differences between values when
    comparing results from on-canvas picking. We get very regular bug
    reports on GIMP about this topic, so the documentation at least should
    give a little more information here.
    Please feel free to review and completely change phrasing or anything,
    if my English is lacking. I am mostly hoping to get the initial sparkle.
    :-)

 src/dialogs/color-dialog.xml | 9 +++++++++
 1 file changed, 9 insertions(+)
---
diff --git a/src/dialogs/color-dialog.xml b/src/dialogs/color-dialog.xml
index 8317a7d75..1e2c365ac 100644
--- a/src/dialogs/color-dialog.xml
+++ b/src/dialogs/color-dialog.xml
@@ -250,6 +250,15 @@
             Instead of picking the colors from the active image, you're able
             to pick colors from the entire screen.
           </para>
+          <para>
+            The downside is that you get colors after they got processed
+            by the full system color stack, in particular color
+            management. It means that the resulting color values may end
+            up different from ones returned by the
+            <link linkend="gimp-tool-color-picker">color picker tool</link>
+            when picking over the canvas. It is up to you to make an
+            informed choice of which color picker to use.
+          </para>
         </listitem>
       </varlistentry>
       <varlistentry id="gimp-colorselector-html">