Drop three things nothing reads, and say what a span makes scalable

`Axis::pair` and `RegionAlign::NEAR` arrived on this branch with no caller
and never got one. `Holds::contains` took `&self` where its five siblings on
the same `Copy` pair of pixels take `self`.

The comment beside a span's cross-axis accumulator said a scalable child
"makes Children scalable too"; `Children` names nothing here, and what it
makes scalable is the span.

src/widget/position/span.rs

@@ -124,7 +124,7 @@ impl Widget for Span {
             if shrinks {
                 // Choosing between a fixed and a relative length from the
                 // span's own eventual width admits multiple fixed points.
-                // A scalable child therefore makes Children scalable too;
+                // A scalable child therefore makes the span scalable too;
                 // only fixed children are compared with one another.
                 if !used.is_px() {
                     ortho = LayoutLen::LEFTOVER;