Picture this round's features in the manual

Four scenes, recorded and looked at frame by frame:
- library-labels: 6, 7, 8 and 9 over four New York frames, 7 again to
  take one off, then the Green chip narrowing the grid and back.
- compose-perspective: two towers shot from below, stood upright with
  Vertical at about +58, then held against Before.
- crop-orphan: a stroke in the top-left corner, a crop that leaves it
  outside, the notice with Undo crop and Keep crop, and Undo crop.
- local-intersect: a linear gradient over the lower half, then
  Intersect and two strokes that survive only where it is.

develop-zoom already ends on hard-edged pixels (previous commit). The
text added is a sentence or two under the existing headings, so the
other branch's structure and anchors are left alone.

docs/manual/README.md

@@ -51,6 +51,8 @@ it reads without telling the colours apart, and the filter bar has a chip for
 each. In develop, the top bar names the open photograph's label and sets it,
 and the same keys work there.
 
+![Labelling four frames with 6, 7, 8 and 9, taking one off with the same key, and filtering the grid to green](media/library-labels.gif)
+
 ### Getting about
 
 Drag the timeline to scrub through years; Ctrl and the wheel resize the
@@ -123,9 +125,11 @@ Hold `Before` to see the photograph as it was.
 ### Looking closer
 
 Double-click for 1:1; drag to move about; double-click again to fit. The
-wheel zooms to any amount in between.
+wheel zooms to any amount in between. Past 1:1 the file's own pixels are
+drawn as hard-edged blocks rather than smoothed, so what you see is what
+the sensor recorded.
 
-![Zooming to 1:1, panning, and back](media/develop-zoom.gif)
+![Zooming to 1:1, panning, then on in with the wheel until the pixels are blocks](media/develop-zoom.gif)
 
 ### Moving between photographs
 
@@ -151,6 +155,17 @@ ratio from the chips. `Done composing` returns to the photograph.
 
 ![Cropping, straightening and choosing a ratio](media/compose.gif)
 
+`Vertical` and `Horizontal`, under `Straighten`, correct the lines that
+converge when the camera is tilted up at a building; the crop refits to
+what is left.
+
+![Standing towers shot from below upright with the Vertical slider](media/compose-perspective.gif)
+
+A crop that leaves a mask wholly outside the frame says so, and offers to
+take the crop back or keep it.
+
+![A stroke in a corner, a crop that leaves it outside, and the notice with Undo crop and Keep crop](media/crop-orphan.gif)
+
 ### Local adjustments
 
 `Local` in the rail turns the column into a mask stack. `Find subjects` runs a
@@ -169,7 +184,11 @@ tinted, as alpha, or as an outline; the eye on its row switches it off.
 ![Looking at the mask three ways, painting into it, then growing its edge](media/local-paint.gif)
 
 Linear and radial gradients, a tone range and a colour range are the other
-ways to make one; each can be combined with any other.
+ways to make one; each can be combined with any other. `∩ Intersect` keeps
+only where the new part and the mask agree: below, a gradient over the
+lower half, then two strokes that survive only where the gradient is.
+
+![A linear gradient, then Intersect and two painted strokes](media/local-intersect.gif)
 
 ### Repair