Improve the crypto_sign man page

Improve the crypto_sign man page

1. Reorder the arguments so that crypto_sign_public_key() comes directly
   after crypto_sign().  This harmonizes the order of the public key
   generation functions with crypto_key_exchange(3monocypher).
2. Move implementation details and complaints about SHA-512 to the
   IMPLEMENTATION DETAILS section.
3. Note that there is no incremental interface available in the
   DESCRIPTION and explain it in the IMPLEMENTATION DETAILS section.
4. Add an example for key generation.  Key generation using straight
   random bytes may come as a surprise to people not used working with
   Curve25519.