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
-`~/.local/share/help/C/org.udpscope.app/`. Two opt-in targets install the manual *alone*, without a
-whole `cmake --install`: `cmake --build build --target install-help-user` writes the same
-`$XDG_DATA_HOME` (default `~/.local/share`) copy the alert describes and needs no root, while
+install command — `cmake --build build --target install-help-user`, the no-root form below, which
+writes the set under `$XDG_DATA_HOME` (default `~/.local/share/help/C/org.udpscope.app/`) and prints
+the path it wrote. Two opt-in targets install the manual *alone*, without a
+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