Update TextStyle API Docs (#83627)

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

@@ -426,6 +426,16 @@ const double _kDefaultFontSize = 14.0;
 ///
 /// ### Inconsistent platform fonts
 ///
+/// By default, fonts differ depending on the platform.
+///
+///  * The default font-family for `Android`,`Fuchsia` and `Linux` is `Roboto`.
+///  * The default font-family for `iOS` is `.SF UI Display`/`.SF UI Text`.
+///  * The default font-family for `MacOS` is `.AppleSystemUIFont`.
+///  * The default font-family for `Windows` is `Segoe UI`.
+//
+// The implementation of these defaults can be found in:
+// /packages/flutter/lib/src/material/typography.dart
+///
 /// Since Flutter's font discovery for default fonts depends on the fonts present
 /// on the device, it is not safe to assume all default fonts will be available or
 /// consistent across devices.