Add doc tests for the Size constructor functions (#7658)

# Objective

Add doc tests for the `Size` constructor functions.

Also changed the function descriptions so they explicitly state the values that elided values are given.

## Changelog
* Added doc tests for the `Size` constructor functions.

crates/bevy_ui/src/geometry.rs

@@ -358,6 +358,17 @@ impl Size {
     }
 
     /// Creates a new [`Size`] where both sides take the given value.
+    ///
+    /// # Example
+    ///
+    /// ```
+    /// # use bevy_ui::{Size, Val};
+    /// #
+    /// let size = Size::all(Val::Px(10.));
+    ///
+    /// assert_eq!(size.width, Val::Px(10.0));
+    /// assert_eq!(size.height, Val::Px(10.0));
+    /// ```
     pub const fn all(value: Val) -> Self {
         Self {
             width: value,
@@ -365,7 +376,18 @@ impl Size {
         }
     }
 
-    /// Creates a new [`Size`] where `width` takes the given value.
+    /// Creates a new [`Size`] where `width` takes the given value and its `height` is `Val::Auto`.
+    ///
+    /// # Example
+    ///
+    /// ```
+    /// # use bevy_ui::{Size, Val};
+    /// #
+    /// let size = Size::width(Val::Px(10.));
+    ///
+    /// assert_eq!(size.width, Val::Px(10.0));
+    /// assert_eq!(size.height, Val::Auto);
+    /// ```
     pub const fn width(width: Val) -> Self {
         Self {
             width,
@@ -373,7 +395,18 @@ impl Size {
         }
     }
 
-    /// Creates a new [`Size`] where `height` takes the given value.
+    /// Creates a new [`Size`] where `height` takes the given value and its `width` is `Val::Auto`.
+    ///
+    /// # Example
+    ///
+    /// ```
+    /// # use bevy_ui::{Size, Val};
+    /// #
+    /// let size = Size::height(Val::Px(10.));
+    ///
+    /// assert_eq!(size.width, Val::Auto);
+    /// assert_eq!(size.height, Val::Px(10.));
+    /// ```
     pub const fn height(height: Val) -> Self {
         Self {
             width: Val::Auto,