EVP_MD_CTX_cleanup replaced with EVP_MD_CTX_reset

The EVP_MD_CTX_cleanup() function was merged into EVP_MD_CTX_reset()
which is called by EVP_MD_CTX_free().  Adjust the documentation to say
that the latter should be used to avoid leaking memory.

CLA: trivial

Reviewed-by: Andy Polyakov <appro@openssl.org>
Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/5921)