Re-add some comment text that was accidentally removed.

cmake/staleness_test_lib.py

@@ -25,6 +25,12 @@
 # (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
 # SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 
+"""Shared code for validating generated_file_staleness_test() rules.
+
+This code is used by test scripts generated from
+generated_file_staleness_test() rules.
+"""
+
 from __future__ import absolute_import
 from __future__ import print_function