layout editor: horizontal-resize icon, divider drag follows the pointer; x caption on every labelled plot; probes and docs

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

help/C/org.udpscope.app/layout.page

@@ -47,7 +47,7 @@
         <gui>+</gui> between two plots, to add a plot to that row. The new plot starts empty.</p>
       </item>
       <item>
-        <p>Drag the arrow on the boundary between two plots to resize them. Neither plot can be
+        <p>Drag the double-headed arrow on the boundary between two plots to resize them. Neither plot can be
         made narrower than about 96 px, and the widths are saved with the session.</p>
       </item>
       <item>