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.

src/help_window.cpp

@@ -77,13 +77,16 @@ void on_help_launched(GObject* source, GAsyncResult* result, gpointer user) {
                 ", so what is missing is a viewer for the `help:` URI it is opened with: install "
                 "one (for example `yelp`) or make it the desktop's default for that scheme.";
     } else {
+      // The system-wide install is named *first* on purpose: a set under the user's data
+      // directory is one a sandboxed viewer (Yelp) cannot read its figures from -- the pages
+      // open without their images -- so the no-root route carries that caveat rather than
+      // leading. The message is kept no longer than the one it replaces; do the same on edit.
       detail += "The manual does not look installed: nothing named \"help:" UDPSCOPE_HELP_NAME
-                "\" was found under $XDG_DATA_HOME/help/<lang>/" UDPSCOPE_HELP_NAME "/ or "
-                "$XDG_DATA_DIRS/help/<lang>/" UDPSCOPE_HELP_NAME "/. Install it with\n\n"
-                "    cmake --build build --target install-help-user\n\n"
-                "which writes the manual where `help:" UDPSCOPE_HELP_NAME "` resolves it under "
-                "your data directory, and prints the path it wrote. If there is no `help:` viewer "
-                "either, install one as well (for example `yelp`).";
+                "\" was found where `help:` resolves. Install it system-wide:\n\n"
+                "    cmake -DCMAKE_INSTALL_PREFIX=/usr .. && sudo cmake --build build --target install-help\n\n"
+                "That writes where the viewer looks. The no-root `install-help-user` writes under "
+                "your data directory instead: a sandboxed viewer shows those pages without their "
+                "figures. A `help:` viewer (for example `yelp`) is needed as well.";
     }
     gtk_alert_dialog_set_detail(alert, detail.c_str());
     gtk_alert_dialog_show(alert, launch->parent);