Add documentation for the OCSP_basic_sign() and OCSP_basic_sign_ctx() functions.

Correct error return value in OCSP_basic_sign().

Reviewed-by: Rich Salz <rsalz@openssl.org>
Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/4190)