help: send readers to the install targets, in the alert and the manual

The failure alert told people to run `cmake --install build --prefix ~/.local`;
that still works, but `cmake --build build --target install-help-user` is the
form that needs no prefix guessing and prints the path it wrote, so the alert
names it and stays its former length. The manual gains a short "Installing
this manual" section on the overview page naming both targets -- the no-root
user one and the prefix-driven one for a system-wide or packaged install,
which the caller wraps in sudo -- and README and the GTK invariants doc follow
the alert's new command.

README.md

@@ -2466,10 +2466,11 @@ alert says **which** thing is missing, because the two have different fixes: `he
 looks for the document under `$XDG_DATA_HOME/help/<lang>/org.udpscope.app/` then
 `$XDG_DATA_DIRS/help/<lang>/org.udpscope.app/`, so with the set installed the alert names the missing
 *viewer* (`yelp`), and without it the alert names the identifier, the expected installed path and the
-install command — `cmake --install build --prefix ~/.local`, which writes
+install command — `cmake --build build --target install-help-user`, the no-root form below, which
-`~/.local/share/help/C/org.udpscope.app/`. Two opt-in targets install the manual *alone*, without a
+writes the set under `$XDG_DATA_HOME` (default `~/.local/share/help/C/org.udpscope.app/`) and prints
-whole `cmake --install`: `cmake --build build --target install-help-user` writes the same
+the path it wrote. Two opt-in targets install the manual *alone*, without a
-`$XDG_DATA_HOME` (default `~/.local/share`) copy the alert describes and needs no root, while
+whole `cmake --install`: `install-help-user` writes that `$XDG_DATA_HOME` (default `~/.local/share`)
+copy and needs no root, while
 `cmake --build build --target install-help` writes to the configured `CMAKE_INSTALL_PREFIX` for a
 packager or a system-wide user to wrap in `sudo`; both print where they wrote. A build tree has
 neither the installed copy nor a

docs/claude/invariants-gtk-and-chrome.md

@@ -159,7 +159,8 @@ Part of [CLAUDE.md](../../CLAUDE.md). These are rules that cost a defect to lear
   `$XDG_DATA_HOME/help/<lang>/<id>/` then `$XDG_DATA_DIRS/help/<lang>/<id>/` for an `index.page` and
   the dialog says which one it is: with the manual there, install a viewer (for example `yelp`); with
   it absent, the identifier, the expected installed path and the install command
-  (`cmake --install build --prefix ~/.local`, which writes `~/.local/share/help/C/<id>/`) — and a
+  (`cmake --build build --target install-help-user`, which installs it under the user data
+  directory; `install-help` is the prefix-wide form for a system or packaged install) — and a
   viewer as well, the second possibility the error alone cannot rule out. Reporting a missing
   *document* as a missing *handler* is the defect that shape avoids.
 

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

@@ -167,4 +167,20 @@
     </list>
   </section>
 
+  <section id="installing">
+    <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>
+    <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>
+    </list>
+  </section>
+
 </page>

src/help_window.cpp

@@ -80,10 +80,10 @@ void on_help_launched(GObject* source, GAsyncResult* result, gpointer user) {
       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 --install build --prefix ~/.local\n\n"
+                "    cmake --build build --target install-help-user\n\n"
-                "which writes ~/.local/share/help/C/" UDPSCOPE_HELP_NAME "/ -- the directory "
+                "which writes the manual where `help:" UDPSCOPE_HELP_NAME "` resolves it under "
-                "`help:" UDPSCOPE_HELP_NAME "` resolves to. If there is no `help:` viewer either, "
+                "your data directory, and prints the path it wrote. If there is no `help:` viewer "
-                "install one as well (for example `yelp`).";
+                "either, install one as well (for example `yelp`).";
     }
     gtk_alert_dialog_set_detail(alert, detail.c_str());
     gtk_alert_dialog_show(alert, launch->parent);