Say window where these comments still say frame

Lengths became lengths of the window when the frame did, and `Part::From`'s
own documentation still described its spans as frame lengths -- which is
what the scroll above read them as.

core/src/ui/place.rs

@@ -5,14 +5,16 @@ use crate::{AxisAlign, Len, PrimitiveHandle, UiRegion, UiSpan};
 pub enum Part {
     /// The whole of it.
     All,
-    /// Frame lengths from where the box starts, which is what a container
-    /// dividing room among its children speaks: a child's report is a length
-    /// of the frame, so the cursor that sums those reports is one too. A
-    /// moved box re-places every child by re-adding its start, exactly.
+    /// Window lengths from where the box starts, which is what a container
+    /// dividing room among its children speaks: a child's report is a window
+    /// 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 [`Self::All`], 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
-    /// length, where saying the same thing in frame lengths would make the
+    /// length, where saying the same thing in window lengths would make the
     /// container read its own box -- and a box chosen from its own answer
     /// then feeds back into the answer.
     Of(UiSpan),