Add doc string

src/test/java/com/google/devtools/build/lib/vfs/bazel/java_opt_binary.bzl

@@ -1,3 +1,23 @@
+# Copyright 2024 The Bazel Authors. All rights reserved.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#    http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+"""
+A wrapper around java_binary that forces it to be built with `-c opt`.
+
+This is useful for benchmark targets.
+"""
+
 load("@with_cfg.bzl", "with_cfg")
 
 java_opt_binary, _java_opt_binary = with_cfg(native.java_binary).set("compilation_mode", "opt").build()