Improve the man page for crypto_key_exchange

Improve the man page for crypto_key_exchange

1. Improve wording in the section on public keys that yield an all-zero
   result.
2. Fix casing (HCHacha20 -> HChacha20).
3. Resolve contraction to keep the manual style.
4. Add example for key generation and key exchange.  It's not quite
   obvious that you just use straight random bytes.  If coming from
   other X25519 implementations, it may be a surprise not having to
   trim the key.  If coming from other public key cryptographic systems,
   it may be a surprise that key generation is this straightforward.