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/README.md

@@ -71,7 +71,9 @@ Drag the timeline to scrub through years; Ctrl and the wheel resize the
 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.
+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.
 
 `Help` in the header, or `F1`, opens the controls and shortcuts: every key and
 gesture, screen by screen, with `See it` beside those this page shows, and