Improve docs for zeroize.c and test_zeroize.gdb
diff --git a/programs/test/zeroize.c b/programs/test/zeroize.c
index 14292b1..d7f2337 100644
--- a/programs/test/zeroize.c
+++ b/programs/test/zeroize.c
@@ -1,5 +1,14 @@
/*
- * Zeroize demonstration program
+ * Zeroize application for debugger-driven testing
+ *
+ * This is a simple test application used for debbuger-driven testing to check
+ * whether calls to mbedtls_zeroize() are being eliminated by compiler
+ * optimizations. This application is used by the GDB script at
+ * tests/scripts/test_zeroize.gdb under the assumption that line numbers do not
+ * change often (as opposed to the library code) because the script sets a
+ * breakpoint at the last return statement in the main() function of this
+ * program. The debugger facilities are then used to manually inspect the
+ * memory and verify that the call to mbedtls_zeroize() was not eliminated.
*
* Copyright (C) 2017, ARM Limited, All Rights Reserved
* SPDX-License-Identifier: Apache-2.0
