Put place before the rel base, and say what a Place decides

An argument that is usually `None` goes last. So `widget_at` and `place_at`
take the place first, and `narrow_rel_base` after it.

`Place` and `Part` also now say which of the two boxes they decide, since
that is the question a caller has to answer to pick between them: `Part` is
the child's region, said as a part of the caller's own, and `Within`/`Fill`
is what becomes of the placement in it.

core/src/ui/place.rs

@@ -1,6 +1,7 @@
 use crate::{AxisAlign, Len, PrimitiveHandle, UiRegion, UiSpan};
 
-/// What of a widget's own box a child is given, along one axis.
+/// A child's region along one axis, said as a part of the region the widget
+/// saying it was given.
 #[derive(Clone, Copy, Debug, PartialEq)]
 pub enum Part {
     /// Window lengths from where the box starts, which is what a container
@@ -8,7 +9,7 @@ pub enum Part {
     /// length, so the cursor that sums those reports is one too. A moved box
     /// re-places every child by re-adding its start, exactly. A fraction
     /// here is a fraction of the window and not of the box -- the whole of a
-    /// box is `Of(UiSpan::FULL)`, not a `rel(1.0)` span.
+    /// box is [`Self::WHOLE`], not a `rel(1.0)` span.
     From(UiSpan),
     /// A part of the box in its own coordinates, which is what a container
     /// that insets one speaks: taking eleven pixels off the end needs no
@@ -42,12 +43,14 @@ impl Part {
     }
 }
 
-/// Where a child goes along one axis, as a part of this widget's box.
+/// A child's region along one axis, and what becomes of its placement in
+/// that region once it has answered.
 #[derive(Clone, Copy, Debug, PartialEq)]
 pub enum Place {
-    /// The child's answer, aligned inside the part by the child's alignment.
+    /// The placement is the child's answer, aligned inside the region by the
+    /// child's alignment.
     Within(Part),
-    /// Exactly the part; the answer is not placed inside it again.
+    /// The region is the placement; the answer is not placed inside it again.
     Fill(Part),
 }
 
@@ -58,8 +61,8 @@ impl Place {
         }
     }
 
-    /// Whether the part is the drawing's box outright, rather than the box
-    /// the answer is placed inside.
+    /// Whether the region is the placement outright, rather than a box the
+    /// answer is placed inside.
     pub(crate) fn fills(self) -> bool {
         matches!(self, Self::Fill(_))
     }