Update documentation on TextPainter to note default color differences (#165048)

This change only changes the documentation of `TextPainter`.

Previously, the documentation was incorrect in stating that the default
text color was white. It changes depending on platform to be contrasted
to the canvas' background color.

This is in an "important" box because it affects apps working on native
& web, and is relatively easy to miss.

It may be worth documenting this elsewhere?

Fixes #165047

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Author: JaffaKetchup

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

@@ -583,8 +583,9 @@ class _LineCaretMetrics {
 /// changes, return to step 2. If the text to be painted changes,
 /// return to step 1.
 ///
-/// The default text style is white. To change the color of the text,
-/// pass a [TextStyle] object to the [TextSpan] in `text`.
+/// The default text style color is white on non-web platforms and black on
+/// the web. If developing across both platforms, always set the text color
+/// explicitly.
 class TextPainter {
   /// Creates a text painter that paints the given text.
   ///