Describe flagging, scrollbars, Help and long steps in the manual

The manual described what the pictures show and missed what this round
changed around them:

- Rating and flagging gave stars only. The keys (0-5, P, X, U), Flag on
  the selection bar, judging in develop without moving on, and the flag
  and stars on the roll's cells had no sentence.
- Nothing said the desktop draws scrollbars on the grid, the sidebar,
  the develop column and Settings, or how the grid's bar is used.
- Nothing said how to open Help: the header's Help or F1, and in
  develop the "?" beside Settings (d9f2596, 380cfda), with its See it
  links and its Manual button.
- Stepping along the roll now goes on past what the roll has loaded
  (a030bfd); Settings gained the duplicate originals line and the
  Manual row.

index.html is regenerated from the README.

docs/manual/index.html

@@ -164,7 +164,13 @@ The bar above the grid filters by rating, flag, colour label and where the
 file is (on this device, or only on the server).</p>
 <h3 id="rating-and-flagging">Rating and flagging</h3>
 <p>Hover a cell and the stars appear; click one. The filter chips above the grid
-count what each rating holds, and clicking <code>3+</code> shows only those.</p>
+count what each rating holds, and clicking <code>3+</code> shows only those. From the
+keyboard, <code>0</code> to <code>5</code> set the stars on the photograph under the pointer or on
+the selection; <code>P</code> picks, <code>X</code> rejects and <code>U</code> takes the flag off, and <code>Flag</code>
+on the selection bar does the same with the pointer. In
+develop the same keys judge the open photograph and stay on it, and the top
+bar carries its stars and <code>Pick</code> and <code>Reject</code>; the roll marks each frame's
+flag and stars.</p>
 <figure><img loading="lazy" src="media/library-rating.gif" alt="Rating two photographs, then filtering the grid to three stars and more"><figcaption>Rating two photographs, then filtering the grid to three stars and more</figcaption></figure>
 <p>Colour labels work as Lightroom's do: <code>6</code> red, <code>7</code> yellow, <code>8</code> green, <code>9</code>
 blue, on the photograph under the pointer or on the selection, and the same
@@ -176,7 +182,13 @@ and the same keys work there.</p>
 <figure><img loading="lazy" src="media/library-labels.gif" alt="Labelling four frames with 6, 7, 8 and 9, taking one off with the same key, and filtering the grid to green"><figcaption>Labelling four frames with 6, 7, 8 and 9, taking one off with the same key, and filtering the grid to green</figcaption></figure>
 <h3 id="getting-about">Getting about</h3>
 <p>Drag the timeline to scrub through years; Ctrl and the wheel resize the
-thumbnails.</p>
+thumbnails. On a desktop a scrollbar beside the grid says how far through the
+library the view is — drag its thumb, or click the track to move a page — and
+the sidebar, the develop column and Settings have one too whenever they run
+past the window. On a tablet they scroll by flick alone.</p>
+<p><code>Help</code> in the header, or <code>F1</code>, opens the controls and shortcuts: every key and
+gesture, screen by screen, with <code>See it</code> beside those this page shows, and
+<code>Manual</code> to open this page. In develop it is the <code>?</code> beside <code>Settings</code>.</p>
 <figure><img loading="lazy" src="media/library-timeline.gif" alt="Scrubbing the timeline"><figcaption>Scrubbing the timeline</figcaption></figure>
 <figure><img loading="lazy" src="media/library-thumbsize.gif" alt="Resizing the thumbnails with Ctrl and the wheel"><figcaption>Resizing the thumbnails with Ctrl and the wheel</figcaption></figure>
 <h3 id="selecting-several">Selecting several</h3>
@@ -252,8 +264,9 @@ the sensor recorded.</p>
 <h3 id="moving-between-photographs">Moving between photographs</h3>
 <p>The roll along the foot of the canvas holds the photographs the grid was
 showing; click one to open it. The right arrow, <code>D</code> or space opens the next,
-and the left arrow or <code>A</code> the one before. The edit on screen is saved on the
-way, so stepping along a shoot loses nothing.</p>
+and the left arrow or <code>A</code> the one before; held down, they go on past the
+stretch the roll has loaded, through everything the grid would show. The
+edit on screen is saved on the way, so stepping along a shoot loses nothing.</p>
 <h3 id="white-balance-from-the-photograph">White balance from the photograph</h3>
 <p>Press <code>pick</code> in the White Balance group, then click something neutral —
 a white wall, a grey card, the air conditioner here. The picker sets the
@@ -332,9 +345,10 @@ set is refused, and the header says so.</p>
 <figure><img loading="lazy" src="media/settings-export.png" alt="Export defaults in Settings"><figcaption>Export defaults in Settings</figcaption></figure>
 <h2 id="settings">Settings</h2>
 <figure><img loading="lazy" src="media/settings.png" alt="The settings page: background activity, indexing, storage, display"><figcaption>The settings page: background activity, indexing, storage, display</figcaption></figure>
-<p>Background activity with progress, thumbnail and face indexing, storage on
-this device, display and colour, export defaults, what is written to XMP
-sidecars, and a diagnostics bundle for a bug report.</p>
+<p>Background activity with progress, thumbnail and face indexing, how many
+duplicate originals the library holds and the way to their review, storage
+on this device, display and colour, export defaults, what is written to XMP
+sidecars, the manual, and a diagnostics bundle for a bug report.</p>
 <h2 id="people">People</h2>
 <p>Face detection and identity run over the library and group faces by person;
 the <code>Identity</code> page is where suggestions are confirmed, rejected and split,