Improve docs to address flutter/flutter#31202 (#31454)

## Description

Clarify that ClipRect can be added to ensure that CustomPainter will be drawing within bounds.

## Related Issues

https://github.com/flutter/flutter/issues/31202

packages/flutter/lib/src/rendering/custom_paint.dart

@@ -157,8 +157,13 @@ abstract class CustomPainter extends Listenable {
   /// coordinate space configured such that the origin is at the top left of the
   /// box. The area of the box is the size of the [size] argument.
   ///
-  /// Paint operations should remain inside the given area. Graphical operations
-  /// outside the bounds may be silently ignored, clipped, or not clipped.
+  /// Paint operations should remain inside the given area. Graphical
+  /// operations outside the bounds may be silently ignored, clipped, or not
+  /// clipped. It may sometimes be difficult to guarantee that a certain
+  /// operation is inside the bounds (e.g., drawing a rectangle whose size is
+  /// determined by user inputs). In that case, consider calling
+  /// [Canvas.clipRect] at the beginning of [paint] so everything that follows
+  /// will be guaranteed to only draw within the clipped area.
   ///
   /// Implementations should be wary of correctly pairing any calls to
   /// [Canvas.save]/[Canvas.saveLayer] and [Canvas.restore], otherwise all

packages/flutter/lib/src/widgets/basic.dart

@@ -419,7 +419,9 @@ class BackdropFilter extends SingleChildRenderObjectWidget {
 /// painters are expected to paint within a rectangle starting at the origin and
 /// encompassing a region of the given size. (If the painters paint outside
 /// those bounds, there might be insufficient memory allocated to rasterize the
-/// painting commands and the resulting behavior is undefined.)
+/// painting commands and the resulting behavior is undefined.) To enforce
+/// painting within those bounds, consider wrapping this [CustomPaint] with a
+/// [ClipRect] widget.
 ///
 /// Painters are implemented by subclassing [CustomPainter].
 ///