Add comment that suppression file is empty

Also add instructions for adding new suppressions in case we need to do
that in the future.

Author: colesbury

Tools/tsan/suppressions.txt

@@ -1,2 +1,10 @@
 # This file contains suppressions for the default (with GIL) build.
-# reference: https://github.com/google/sanitizers/wiki/ThreadSanitizerSuppressions
+#
+# Reference: https://github.com/google/sanitizers/wiki/ThreadSanitizerSuppressions
+#
+# When adding a suppression, include a comment referencing a GitHub issue
+# that describes how to reproduce the race and includes the relevant TSan
+# output.
+
+# There are currently no active suppressions. This file is kept so tooling
+# that expects it can still find it.

Tools/tsan/suppressions_free_threading.txt

@@ -1,5 +1,12 @@
-# This file contains suppressions for the free-threaded build. It contains the
-# suppressions for the default build and additional suppressions needed only in
+# This file contains suppressions for the free-threaded build. It includes the
+# default build suppressions plus any additional suppressions needed only in
 # the free-threaded build.
 #
-# reference: https://github.com/google/sanitizers/wiki/ThreadSanitizerSuppressions
+# Reference: https://github.com/google/sanitizers/wiki/ThreadSanitizerSuppressions
+#
+# When adding a suppression, include a comment referencing a GitHub issue
+# that describes how to reproduce the race and includes the relevant TSan
+# output.
+
+# There are currently no active suppressions. This file is kept so tooling
+# that expects it can still find it.