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.

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"
-                "which writes ~/.local/share/help/C/" UDPSCOPE_HELP_NAME "/ -- the directory "
-                "`help:" UDPSCOPE_HELP_NAME "` resolves to. If there is no `help:` viewer either, "
-                "install one as well (for example `yelp`).";
+                "    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`).";
     }
     gtk_alert_dialog_set_detail(alert, detail.c_str());
     gtk_alert_dialog_show(alert, launch->parent);