Clean up the docstring for ColorFilter.matrix (#46298)

The first line is important since it is pulled out into https://api.flutter.dev/flutter/dart-ui/ColorFilter-class.html.  The existing wording is very confusing about what the input actually is.

[C++, Objective-C, Java style guides]: https://github.com/flutter/engine/blob/main/CONTRIBUTING.md#style

lib/ui/painting.dart

@@ -3482,8 +3482,9 @@ class ColorFilter implements ImageFilter {
         _matrix = null,
         _type = _kTypeMode;
 
-  /// Construct a color filter that transforms a color by a 5x5 matrix, where
-  /// the fifth row is implicitly added in an identity configuration.
+  /// Construct a color filter from a 4x5 row-major matrix. The matrix is
+  /// interpreted as a 5x5 matrix, where the fifth row is the identity
+  /// configuration.
   ///
   /// Every pixel's color value, represented as an `[R, G, B, A]`, is matrix
   /// multiplied to create a new color: