Update comments for net/data/verify_certificate_chain_unittest/

net/data/verify_certificate_chain_unittest/target-signed-using-ecdsa/generate-chains.py

 #!/usr/bin/python
 # Copyright (c) 2015 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
-"""Certificate chain with a trusted root using RSA, and intermediate using EC,
-and a target certificate using RSA. Verification is expected to succeed."""
+"""Certificate chain where the root certificate holds an RSA key, intermediate
+certificate holds an EC key, and target certificate holds an RSA key. The
+target certificate has a valid signature using ECDSA."""
 
 import sys
 sys.path += ['..']
 
 import common
 
-# Self-signed root certificate (used as trust anchor). using RSA.
+# Self-signed root certificate using an RSA key.
 root = common.create_self_signed_root_certificate('Root')
 
 # Intermediate using an EC key for the P-384 curve.
 intermediate = common.create_intermediate_certificate('Intermediate', root)
 intermediate.set_key(common.get_or_generate_ec_key(
     'secp384r1', common.create_key_path(intermediate.name)))
 
 # Target certificate contains an RSA key (but is signed using ECDSA).
 target = common.create_end_entity_certificate('Target', intermediate)
 
 chain = [target, intermediate, root]
 common.write_chain(__doc__, chain, 'chain.pem')