docs: correct the shortcut and help entry points, and the rate example

The whole-branch review found README describing a toolbar "?" button that
does not exist, Ctrl+? for an overview bound to Ctrl+/, bare `e` for an
export bound to Ctrl+E, and a help window that opens "on the page nearest
to what the user was doing" when it unconditionally selects Overview. All
four would send a reader looking for something that is not there. The key
table also claimed to be full while omitting F1, Ctrl+/ and Ctrl+Q, and
did not say that `f` is the plot's own key rather than an app accelerator.

format_rate() is %.3g, so it prints "10 kS/s"; the two doc comments in
status_text.hpp and the example in help page 01 still said "10.0 kS/s" —
the same error already corrected in 14-status.txt and CLAUDE.md, left in
the header that declares the function and in a second user-facing page.

close_help_windows()'s comment described only gtk_window_close(), and
show_shortcuts()'s called the overview "hand-rolled"; the Adwaita rework
made one half-true and the other false.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/help_window.hpp

@@ -29,12 +29,14 @@ struct HelpReport {
 HelpReport help_self_check();
 
 /// The shortcut overview: the application accelerators and the plot's own interactions,
-/// in the sections the tables name. Hand-rolled because GtkShortcutsWindow is deprecated
-/// as of GTK 4.18 and this project builds warning-free.
+/// in the sections the tables name. An AdwShortcutsDialog, because GtkShortcutsWindow is
+/// deprecated as of GTK 4.18 and this project builds warning-free.
 void show_shortcuts(GtkWindow* parent);
 
-/// Closes whichever of the three windows are open, via gtk_window_close() so the
-/// close-request handlers run and the pointers are nulled. Used by the probe.
+/// Closes whichever of the three windows are open. The About dialog and the help window are
+/// GtkWindows and close with gtk_window_close(), so their close-request handlers run; the
+/// shortcut overview is an AdwDialog and closes with adw_dialog_close(), which emits ::closed.
+/// Either way the file-scope pointer is nulled by the handler. Used by the probe.
 void close_help_windows();
 
 }  // namespace udpscope