[gimp-help] Improve Adding layer mask documentation

[Jacob Boerema <jboerema src gnome org>]

commit 4e8a134c256333121d302eddb6e1e1332358bce9
Author: Jacob Boerema <jgboerema gmail com>
Date:   Sun May 29 16:39:53 2022 -0400

    Improve Adding layer mask documentation
    
    Closes #222 by explicitly mentioning that the Channels here do not include
    the default RGBA channels.

 src/menus/layer/mask-add.xml | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)
---
diff --git a/src/menus/layer/mask-add.xml b/src/menus/layer/mask-add.xml
index d2129274f..ec855e8ba 100644
--- a/src/menus/layer/mask-add.xml
+++ b/src/menus/layer/mask-add.xml
@@ -155,7 +155,9 @@
                 <para>
                   With this option the layer mask is initialized with a
                   selection mask you have created before, stored in the
-                  Channel dialog.
+                  Channel dialog. Note: that the normal RGBA channels
+                  are not part of this list, it only lists the extra
+                  channels you have created.
                 </para>
               </listitem>
             </varlistentry>