Fix DynamicSpacing docs (#20509)

In #20504 the CustomSpacing enum variants ended up not having docs. This
PR fixes that, now docs correctly show for variants.


https://github.com/user-attachments/assets/8cc409c9-7b71-4c21-a538-3fd5dded3e00

Release Notes:

- N/A

crates/ui/src/styles/spacing.rs

@@ -4,6 +4,29 @@ use theme::{ThemeSettings, UiDensity};
 use ui_macros::derive_dynamic_spacing;
 
 // Derives [DynamicSpacing]. See [ui_macros::derive_dynamic_spacing].
+//
+// There are 3 UI density settings: Compact, Default, and Comfortable.
+//
+// When a tuple of three values is provided, the values are used directly.
+//
+// Example: (1, 2, 4) => Compact: 1px, Default: 2px, Comfortable: 4px
+//
+// When a single value is provided, the standard spacing formula is
+// used to derive the of spacing values. This formula can be found in
+// the macro.
+//
+// Example:
+//
+// Assuming the standard formula is (n-4, n, n+4)
+//
+// 24 => Compact: 20px, Default: 24px, Comfortable: 28px
+//
+// The [DynamicSpacing] enum variants use a BaseXX format,
+// where XX = the pixel value @ default rem size and the default UI density.
+//
+// Example:
+//
+// DynamicSpacing::Base16 would return 16px at the default UI scale & density.
 derive_dynamic_spacing![
     (0, 0, 0),
     (1, 1, 2),