Size: add some documentation comments (NFC)

Add some documentation comments and region markers to Size.

Author: compnerd

Sources/SwiftWin32/CG/Size.swift

@@ -1,16 +1,40 @@
 // Copyright © 2019 Saleem Abdulrasool <[email protected]>
 // SPDX-License-Identifier: BSD-3-Clause
 
+/// A structure that contains width and height values.
 public struct Size {
-  public static let zero: Size = Size(width: 0, height: 0)
+  // MARK - Geometric Properties
 
-  public var height: Double
+  /// A width value.
   public var width: Double
 
-  public init(width: Float, height: Float) {
-    self.init(width: Double(width), height: Double(height))
+  /// A height value.
+  public var height: Double
+
+  // MARK - Special Values
+
+  /// The size whose width and height are both zero.
+  public static var zero: Size {
+    Size(width: 0, height: 0)
   }
 
+  /// Creates a size with zero width and height.
+  public init() {
+    self = .zero
+  }
+
+  // MARK - Transforming Sizes
+
+  /// Returns the height and width resulting from a transformation of an
+  /// existing height and width.
+  func applying(_ transform: AffineTransform) -> Size {
+    return Size(width: transform.a * self.width + transform.c * self.height,
+                height: transform.b * self.width + transform.d * self.height)
+  }
+
+  // MARK - Initializers
+
+  /// Creates a size with dimensions specified as floating-point values.
   public init(width: Double, height: Double) {
     self.height = height
     self.width = width
@@ -19,11 +43,6 @@ public struct Size {
   public init(width: Int, height: Int) {
     self.init(width: Double(width), height: Double(height))
   }
-
-  func applying(_ transform: AffineTransform) -> Size {
-    return Size(width: transform.a * self.width + transform.c * self.height,
-                height: transform.b * self.width + transform.d * self.height)
-  }
 }
 
 extension Size: Equatable {