Update documentation for Poly1305-ChaCha20 AEAD, RFC-7539
* cipher/cipher-poly1305.c: Add RFC-7539 to header. * doc/gcrypt.texi: Update Poly1305 AEAD documentation with mention of RFC-7539; Drop Salsa from supported stream ciphers for Poly1305 AEAD.
- Signed-off-by: Jussi Kivilinna <jussi.kivilinna@iki.fi>