Update comments/docstrings in TestGenerator

Signed-off-by: Werner Lewis <werner.lewis@arm.com>

scripts/mbedtls_dev/test_generation.py

@@ -129,11 +129,13 @@ class BaseTarget(metaclass=ABCMeta):
 
 
 class TestGenerator:
-    """Generate test data."""
+    """Generate test cases and write to data files."""
     def __init__(self, options) -> None:
         self.test_suite_directory = self.get_option(options, 'directory',
                                                     'tests/suites')
-        # Add file Targets which have been declared in other modules
+        # Update `targets` with an entry for each child class of BaseTarget.
+        # Each entry represents a file generated by the BaseTarget framework,
+        # and enables generating the .data files using the CLI.
         self.targets.update({
             subclass.target_basename: subclass.generate_tests
             for subclass in BaseTarget.__subclasses__()