Add usage info of generate_psa_constants script
#!/usr/bin/env python3
-"""Generate programs/psa/psa_constant_names_generated.c
+"""Generate psa_constant_names_generated.c
which is included by programs/psa/psa_constant_names.c.
The code generated by this module is only meant to be used in the context
of that program.
+
+An argument passed to this script will modify the output directory where the
+file is written:
+* by default (no arguments passed): writes to programs/psa/
+* OUTPUT_FILE_DIR passed: writes to OUTPUT_FILE_DIR/
"""
import os
