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>

README.md

@@ -1220,7 +1220,7 @@ commit and timestamp come from `src/build_info.cpp`, generated by CMake at confi
 regenerated on every build — so the commit and the timestamp always belong to the binary that is
 running, and `git` absent leaves the commit as `"unknown"` without breaking the build.
 
-**The shortcut overview** (toolbar **?** button, or Ctrl+?) lists all keyboard and mouse controls
+**The shortcut overview** (overflow menu, or Ctrl+/) lists all keyboard and mouse controls
 in two groups — application actions and plot controls — and is an `AdwShortcutsDialog` rather
 than GTK4's deprecated `GtkShortcutsWindow`. libadwaita ≥ 1.8 is therefore a hard dependency;
 `CLAUDE.md` records the one consequence worth knowing before editing (the stylesheet it installs
@@ -1236,8 +1236,11 @@ reaches every widget). The full key table:
 | `g` | show / hide math channels |
 | `l` | show / hide the layout editor |
 | `s` | show / hide the side panel |
-| `f` | fit to data |
-| `e` | export visible range |
+| `f` | fit to data (the plot's own key, not an application accelerator) |
+| `Ctrl+E` | export visible range |
+| `F1` | open the help window |
+| `Ctrl+/` | open the shortcut overview |
+| `Ctrl+Q` | quit |
 | `+` / `-` | zoom in / out (both axes) |
 | `Ctrl+` / `Ctrl-` | zoom in / out (x axis only) |
 | `Alt+` / `Alt-` | zoom in / out (y axis only) |
@@ -1252,8 +1255,8 @@ The six "Plot mouse" rows (wheel, drag, right-click) carry no accelerator key
 syntax has no member for pointer gestures — and put the prose in the `subtitle` field instead,
 which `AdwShortcutsItem` renders as free text.
 
-**The 14-page help window** (overflow menu, F1, or the toolbar **?** button long-press) is a
-scrollable text view that opens on the page nearest to what the user was doing: the help pages
+**The 14-page help window** (overflow menu or F1) is a scrollable text view that opens on the
+Overview page, so it never comes up blank: the help pages
 are compiled into the binary as a GResource (`src/help/udpscope_help.gresource.xml`,
 `--c-name=udpscope_help`) for the same reason the icons are — udpscope is run straight out of the
 build tree with no install step, so a search path would have to be found at runtime and a missing