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/01-overview.txt

@@ -23,4 +23,4 @@ one on in the side panel (<tt>s</tt>) and drag it onto a row.
 <b>Example</b>
 
 A 3-signal source at 10 kS/s with a 200 MB buffer holds 1677 s of history. Enable all three,
-drop them on row 1, and the status bar reads <tt>● live · 10.0 kS/s · 2 s window</tt>.
+drop them on row 1, and the status bar reads <tt>● live · 10 kS/s · 2 s window</tt>.