fix: add information on PKCS1v15 (GoogleCloudPlatform#3236)

Author: frankyn

storage/signed_urls/generate_signed_urls.py

@@ -109,6 +109,7 @@ def generate_signed_url(http://github.com/service_account_file,%20bucket_name,%20object_name,%3C/div%3E%3C/code%3E%3C/div%3E%3C/td%3E%3C/tr%3E%3Ctr%20class=%22diff-line-row%22%3E%3Ctd%20data-grid-cell-id=%22diff-80855396bdf3cadee603990a0bbe81d4def29d8a4e3db71fed17db782c36add3-109-109-0%22%20data-selected=%22false%22%20role=%22gridcell%22%20style=%22background-color:var(--bgColor-default);text-align:center" tabindex="-1" valign="top" class="focusable-grid-cell diff-line-number position-relative diff-line-number-neutral left-side">109
                                 canonical_request_hash])
 
+    # signer.sign() signs using RSA-SHA256 with PKCS1v15 padding
     signature = binascii.hexlify(
         google_credentials.signer.sign(string_to_sign)
     ).decode()