Clarify the descriptions of test-case-data-related modules

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
diff --git a/scripts/mbedtls_dev/test_data_generation.py b/scripts/mbedtls_dev/test_data_generation.py
index a88425f..f8e86ed 100644
--- a/scripts/mbedtls_dev/test_data_generation.py
+++ b/scripts/mbedtls_dev/test_data_generation.py
@@ -1,4 +1,7 @@
-"""Common test generation classes and main function.
+"""Common code for test data generation.
+
+This module defines classes that are of general use to automatically
+generate .data files for unit tests, as well as a main function.
 
 These are used both by generate_psa_tests.py and generate_bignum_tests.py.
 """