Explain hairline rendering in BorderSide.width docs (#33226)

packages/flutter/lib/src/painting/borders.dart

@@ -26,6 +26,9 @@ enum BorderStyle {
 /// A [Border] consists of four [BorderSide] objects: [Border.top],
 /// [Border.left], [Border.right], and [Border.bottom].
 ///
+/// Note that setting [BorderSide.width] to 0.0 will result in hairline
+/// rendering. A more involved explanation is present in [BorderSide.width].
+///
 /// {@tool sample}
 ///
 /// This sample shows how [BorderSide] objects can be used in a [Container], via
@@ -103,9 +106,15 @@ class BorderSide {
   /// The color of this side of the border.
   final Color color;
 
-  /// The width of this side of the border, in logical pixels. A
-  /// zero-width border is a hairline border. To omit the border
-  /// entirely, set the [style] to [BorderStyle.none].
+  /// The width of this side of the border, in logical pixels.
+  ///
+  /// Setting width to 0.0 will result in a hairline border. This means that
+  /// the border will have the width of one physical pixel. Also, hairline
+  /// rendering takes shortcuts when the path overlaps a pixel more than once.
+  /// This means that it will render faster than otherwise, but it might
+  /// double-hit pixels, giving it a slightly darker/lighter result.
+  ///
+  /// To omit the border entirely, set the [style] to [BorderStyle.none].
   final double width;
 
   /// The style of this side of the border.
@@ -143,7 +152,7 @@ class BorderSide {
   ///
   /// Since a zero width is normally painted as a hairline width rather than no
   /// border at all, the zero factor is special-cased to instead change the
-  /// style no [BorderStyle.none].
+  /// style to [BorderStyle.none].
   ///
   /// Values for `t` are usually obtained from an [Animation<double>], such as
   /// an [AnimationController].