Say that a tablet's scrollers show a position cue

The manual said the lists on a tablet scroll by flick alone, and
outstanding.md §4a that Android had no scrollbars by design. Both now
describe the cue from #69: a thin line while the view moves, gone once
it stops, that a flick starting on it passes through; and §4a notes
DR_SCROLL_CUE for looking at it on a desktop, and that it is not yet
checked on the tablet. The bundled manual is regenerated to match.

docs/manual/index.html

@@ -191,7 +191,9 @@ and the same keys work there.</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>
+past the window. On a tablet they scroll by flick alone, and a thin line at
+the right-hand edge shows where the view is while it moves, fading once it
+stops; it is only a picture, and a flick that starts on it scrolls the list.</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>