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/status_text.hpp

@@ -46,7 +46,7 @@ struct StatusSnapshot {
   int reconnects = 0;
 };
 
-/// The bar: "live · 10.0 kS/s · 2 s window", plus " · event 4.2 s ago" in trigger mode and
+/// The bar: "live · 10 kS/s · 2 s window", plus " · event 4.2 s ago" in trigger mode and
 /// nothing else ever. The badge glyph is a separate widget and is not part of this string.
 std::string format_glance(const StatusSnapshot& s);
 
@@ -55,7 +55,8 @@ std::string format_glance(const StatusSnapshot& s);
 /// their order must not change without updating it.
 std::string format_detail(const StatusSnapshot& s);
 
-/// A sample rate as "10.0 kS/s". Moved here from ui.cpp, which still calls it.
+/// A sample rate as "10 kS/s" — `%.3g`, so a trailing zero is dropped and the field's width
+/// cannot track the stream. Moved here from ui.cpp, which still calls it.
 std::string format_rate(double hz);
 
 /// An elapsed time, stepping to a coarser unit rather than growing digits, so its width