[gimp-help] Update Layer Threshold Alpha documentation

[Jacob Boerema <jboerema src gnome org>]

commit 9be9541bd0e80b099e2a7ec94ca2beef1876aa12
Author: Jacob Boerema <jgboerema gmail com>
Date:   Wed Oct 19 14:39:47 2022 -0400

    Update Layer Threshold Alpha documentation
    
    This filter was changed to a GEGL filter long ago, but so far never
    updated.
    
    (cherry picked from commit 3d79a46c61a1982f756757dea691f5627bd43615)

 images/C/menus/layer/threshold-alpha.png | Bin 13543 -> 15355 bytes
 src/menus/layer/threshold-alpha.xml      |  41 ++++++++++++++++++-------------
 src/menus/layer/transparency.xml         |   2 +-
 3 files changed, 25 insertions(+), 18 deletions(-)
---
diff --git a/images/C/menus/layer/threshold-alpha.png b/images/C/menus/layer/threshold-alpha.png
index 813b91db9..e843c618c 100644
Binary files a/images/C/menus/layer/threshold-alpha.png and b/images/C/menus/layer/threshold-alpha.png differ
diff --git a/src/menus/layer/threshold-alpha.xml b/src/menus/layer/threshold-alpha.xml
index f7a826917..cfc85db10 100644
--- a/src/menus/layer/threshold-alpha.xml
+++ b/src/menus/layer/threshold-alpha.xml
@@ -6,7 +6,7 @@
   2007-03-12 Kolbjørn: added no translation
   2005/11/16 split of from layer-transparency.xml by axel.wernicke
 -->
-<sect2 id="plug-in-threshold-alpha">
+<sect2 xmlns:xi="http://www.w3.org/2001/XInclude"; id="gimp-filter-threshold-alpha">
   <title>Threshold Alpha</title>
 
   <indexterm>
@@ -21,12 +21,11 @@
   <para>
     The <guimenuitem>Threshold Alpha</guimenuitem> command converts
     semi-transparent areas of the active layer into completely transparent or
-    completely opaque areas, based on a threshold you set, between 0 and
-    255. It only works on layers of RGB images which have an alpha
-    channel. If the image is Grayscale or Indexed, or if the layer does not
-    have an alpha channel, the menu entry is insensitive and grayed out.
-    If the <guilabel>Keep transparency</guilabel> option is checked in the
-    Layers dialog, the command displays an error message.
+    completely opaque areas, based on a threshold you set. It only works on
+    layers which have an alpha channel.
+    If the <guilabel>Lock alpha channel</guilabel> option is checked in the
+    <link linkend="gimp-layer-dialog">Layers dialog</link>, the filter will
+    have no effect.
   </para>
 
   <sect3>
@@ -34,11 +33,11 @@
     <itemizedlist>
       <listitem>
         <para>
-          You can access this command from the image menubar through
+          You can access this command from the menubar through
           <menuchoice>
             <guimenu>Layer</guimenu>
             <guisubmenu>Transparency</guisubmenu>
-            <guimenuitem>Threshold Alpha</guimenuitem>
+            <guimenuitem>Threshold Alpha...</guimenuitem>
           </menuchoice>.
         </para>
       </listitem>
@@ -49,7 +48,7 @@
     <title>Description of the Dialog Window</title>
     <figure>
       <title>
-        The only one option of the <quote>Threshold Alpha</quote> dialog
+        The <quote>Threshold Alpha</quote> filter options dialog
       </title>
       <mediaobject>
         <imageobject>
@@ -60,15 +59,23 @@
     </figure>
     <variablelist>
       <varlistentry>
-        <term>Threshold</term>
+        <term>Presets, Input Type, Blending Options, Preview, Split view</term>
+        <listitem>
+          <xi:include href="../../filters/about-common-features.xml"/>
+        </listitem>
+      </varlistentry>
+
+      <varlistentry>
+        <term>Value</term>
         <listitem>
           <para>
-            You can set the transparency value to be used as a threshold by
-            using the slider or by entering a value between 0 and 255 in
-            the input box. All transparency values above this threshold will
-            become opaque and all transparency values below or equal to this
-            threshold will become completely transparent. The transition is
-            abrupt.
+            The threshold alpha value determines which values will become
+            transparent or opaque.
+            You can set the transparency value to be used by using the slider
+            or by entering a value between 0 and 1 in the input box.
+            All transparency values above this threshold will become opaque
+            and all transparency values below or equal to this threshold will
+            become completely transparent.
           </para>
           <note>
             <para>
diff --git a/src/menus/layer/transparency.xml b/src/menus/layer/transparency.xml
index d15fbe74f..1528c9c5e 100644
--- a/src/menus/layer/transparency.xml
+++ b/src/menus/layer/transparency.xml
@@ -78,7 +78,7 @@
       </listitem>
       <listitem>
         <para>
-          <xref linkend="plug-in-threshold-alpha"/>
+          <xref linkend="gimp-filter-threshold-alpha"/>
         </para>
       </listitem>
       <listitem>