Clarified that the signature's buffer size, `s`, is not used as an

IN parameter.

Under the old docs, the only thing stated was "at most
EVP_PKEY_size(pkey) bytes will be written". It was kind of misleading
since it appears EVP_PKEY_size(pkey) WILL be written regardless of the
signature's buffer size.

(cherry picked from commit 6e6ba36d)