Make the matchesGoldenFile docs link to an explanation of how to create golden image files (#31093)

packages/flutter_test/lib/src/matchers.dart

diff --git a/packages/flutter_test/lib/src/matchers.dart b/packages/flutter_test/lib/src/matchers.dart
index 47aca19..098d4c8 100644
--- a/packages/flutter_test/lib/src/matchers.dart
+++ b/packages/flutter_test/lib/src/matchers.dart
@@ -297,6 +297,9 @@
 /// await expectLater(imageFuture, matchesGoldenFile('save.png'));
 /// ```
 ///
+/// Golden image files can be created or updated by running `flutter test
+/// --update-goldens` on the test.
+///
 /// See also:
 ///
 ///  * [goldenFileComparator], which acts as the backend for this matcher.