Fix SSL_CTX_set_session_id_context() docs

Also, use define rather than sizeof

Reviewed-by: Tomas Mraz <tmraz@fedoraproject.org>
Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/9377)

(cherry picked from commit fe9edc9d39c96c965efc4fde12ddf7fa8a852025)