docs: add diff_test note to generated_file_test

fixes #3184

docs/Built-ins.md

@@ -1676,6 +1676,10 @@ Tests that a file generated by Bazel has identical content to a file in the work
 This is useful for testing, where a "snapshot" or "golden" file is checked in,
 so that you can code review changes to the generated output.
 
+Note, for binary files you can just use the `diff_test` rule from bazel-skylib.
+See https://github.com/bazelbuild/bazel-skylib/blob/main/docs/diff_test_doc.md
+and https://blog.aspect.dev/bazel-can-write-to-the-source-folder
+
 
 **PARAMETERS**
 

internal/generated_file_test/generated_file_test.bzl

@@ -8,6 +8,10 @@ def generated_file_test(name, generated, src, substring_search = False, src_dbg
     This is useful for testing, where a "snapshot" or "golden" file is checked in,
     so that you can code review changes to the generated output.
 
+    Note, for binary files you can just use the `diff_test` rule from bazel-skylib.
+    See https://github.com/bazelbuild/bazel-skylib/blob/main/docs/diff_test_doc.md
+    and https://blog.aspect.dev/bazel-can-write-to-the-source-folder
+
     Args:
         name: Name of the rule.
         generated: a Label of the output file generated by another rule