Add SSL_shutdown to SSL_get_error's documentation

SSL_shutdown can fail if called during initialization, and in such case, it'll
add an error to the error queue. This adds SSL_shutdown to the list of functions
that should preceed the call to SSL_get_error.

CLA: trivial

Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Tomas Mraz <tmraz@fedoraproject.org>
(Merged from https://github.com/openssl/openssl/pull/7465)

(cherry picked from commit df9fd168ceb1f187365d24fef829d6d48b1077a9)