Clarify how the EVP IV is used with ChaCha

Clarify how the EVP IV is used with ChaCha

EVP_chacha20() was aligned to follow OpenSSL's nonconformant implementation
during a2k20 by djm and myself in an effort to allow OpenSSH to use the
OpenSSL 1.1 API. Some corresponding OpenSSL 1.1 documentation was imported
at the same time. A comment attempted to translate between implementation
and the incorrect documentation, which was necessarily gibberish. Improve
the situation by rephrasing and dropping nonsensical bits.

Prompted by a question of schwarze