Clarify function argument to crypto_x25519

Clarify function argument to crypto_x25519

1. Change param "shared_secret" in crypto_x25519 to "raw_shared_secret"
   to aid quick identification of the difference between it and
   crypto_key_exchange; the "key" vs. "secret" gets lost easily because
   of the shared prefix "shared_".
   This change was traced everywhere in the source code where the old
   name was used as well as the man page.
2. Fix not having punctuation as a separate word in man page macro.