message: Add RSA-SSA-PKCS1 to comment in signature
diff: // Package signature provides implementations of the Signer and Verifier
  // primitives.
  //
  -// To sign data using Tink you can use ECDSA or ED25519 key templates.
  +// To sign data using Tink you can use ECDSA, ED25519 or RSA-SSA-PKCS1 key templates.
  package signature
  import (
  