Update secret.rst (#516)
Update secret.rst (#516)

As described in https://libsodium.gitbook.io/doc/public-key_cryptography/authenticated_encryption#algorithm-details, libsodium uses XSalsa20 instead of Salsa20 for its underlying encryption, also update the link to point to the libsodium docs about XSalsa20 for reference
pyca/pynacl,lmctv/pynacl,lmctv/pynacl,pyca/pynacl,reaperhulk/pynacl,pyca/pynacl,lmctv/pynacl,pyca/pynacl,reaperhulk/pynacl,lmctv/pynacl,lmctv/pynacl,reaperhulk/pynacl,reaperhulk/pynacl,reaperhulk/pynacl,pyca/pynacl