Document MBEDTLS_TEST_DEPRECATED
diff --git a/tests/suites/main_test.function b/tests/suites/main_test.function
index 6ddfc59..a1ba610 100644
--- a/tests/suites/main_test.function
+++ b/tests/suites/main_test.function
@@ -29,6 +29,11 @@
#include "psa/crypto.h"
#endif /* MBEDTLS_USE_PSA_CRYPTO */
+/* Test code may use deprecated identifiers only if the preprocessor symbol
+ * MBEDTLS_TEST_DEPRECATED is defined. When building tests, set
+ * MBEDTLS_TEST_DEPRECATED explicitly if MBEDTLS_DEPRECATED_WARNING is
+ * enabled but the corresponding warnings are not treated as errors.
+ */
#if !defined(MBEDTLS_DEPRECATED_REMOVED) && !defined(MBEDTLS_DEPRECATED_WARNING)
#define MBEDTLS_TEST_DEPRECATED
#endif
