crypto_ietf_chacha20: note nonce overflow handling

crypto_ietf_chacha20: note nonce overflow handling

IETF ChaCha20 has a 32-bit counter.
This means a practical limit of 256 GiB of data for each nonce.
Additionally, IETF QUIC seems to require being able to handle
0xffffffff (I-D.draft-ietf-quic-tls-33 § 5.4.4),
thus getting very close to the overflow,
though not triggering it.

Unlike libsodium and other libraries, we do not have the option to
panic and take down whatever process is running the code and triggering
the overflow condition because Monocypher is neither allowed to use the C
standard library nor allowed to invoke undefined behavior to cause a
crash;
the applicable RFC provides no guidance what to do in this case, either.

Therefore, staying within the (nonce, counter) limits is necessarily
application responsibility;
it is an invariant that, when voided, Monocypher is allowed to do
anything,
similar to the non-guarantee we make for the crypto_blake2b family
and the crypto_argon2i family.

While already here, fix the wrong function prototype in the synopsis.