help: recommend the system-wide install and record the location trap

A Mallard set under $XDG_DATA_HOME/help is one Yelp cannot draw the figures
of: its sandbox exposes the system help directories but not the user's data
directory, and each figure is a separate file the page references. The pages
open without their images, and through the app's help: URI the viewer spins
for ever because the failed image fetch means the page's load event never
fires -- which reads as a navigation failure, not a missing resource.

The alert and the manual both recommended install-help-user as the no-root
route, so they led users into exactly that case. Name the system-wide install
first and state the user-local caveat where the advice is given, and record
the finding in the GTK traps invariant so the next figure that will not draw
is not chased as a markup or a file problem.

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

@@ -171,15 +171,21 @@
     <title>Installing this manual</title>
     <p>The manual is a help set the desktop viewer reads off disk, so <key>F1</key> opens it only
     once a copy sits where <cmd>help:</cmd> resolves. Either of two targets installs it on its own,
-    without building the application, and each prints where it wrote:</p>
+    without building the application, and each prints where it wrote. Prefer the system-wide one:
+    it is the copy every viewer can read, and the reason is worth knowing before you pick the
+    other:</p>
     <list>
-      <item><p>For your own account, needing no root:
-      <cmd>cmake --build build --target install-help-user</cmd> writes it under
-      <file>$XDG_DATA_HOME</file> (default <file>~/.local/share</file>).</p></item>
       <item><p>For a system-wide or packaged install:
       <cmd>cmake --build build --target install-help</cmd> writes it under the configured
-      <cmd>CMAKE_INSTALL_PREFIX</cmd>, so point that at <file>/usr</file> and wrap the target in
-      <cmd>sudo</cmd>.</p></item>
+      <cmd>CMAKE_INSTALL_PREFIX</cmd>, so point that at <file>/usr</file>
+      (<cmd>cmake -DCMAKE_INSTALL_PREFIX=/usr ..</cmd>) and wrap the target in
+      <cmd>sudo</cmd>. A viewer reads its figures as separate files, and its sandbox exposes the
+      system help directories, so this copy always shows them.</p></item>
+      <item><p>For your own account, needing no root:
+      <cmd>cmake --build build --target install-help-user</cmd> writes it under
+      <file>$XDG_DATA_HOME</file> (default <file>~/.local/share</file>). Be aware that the same
+      sandbox does <em>not</em> expose the user's data directory: those pages open, but a viewer
+      such as Yelp shows them without their figures.</p></item>
     </list>
   </section>