doc/manual: Cleanup selection_border_width doc

We will explicitly mention platforms where the border width can't be
changed once we have them. Here we imply the selection provided by
the desktop, which we will probably use for Wayland support.

doc/manual.md

@@ -342,9 +342,7 @@ settings file:
     Depending on the platform, the final width can be scaled
     proportionally to DPI, which may be the physical DPI of the
     display, the virtual DPI set via global GUI/font scale settings,
-    or a combination of both. On some platforms, it may be impossible
-    to change the border width at all, in which case this option will
-    have no effect.
+    or a combination of both.
 
 *   `ui_tray_icon_visible` (`true` by default) whether to show an icon
     in the notification area.