SSL: Document SSL_add_{file,dir,store}_cert_subjects_to_stack()

This also removes the incorrect documentation comments by those
functions, and fixes a bug in SSL_add_store_cert_subjects_to_stack(),
where the condition for recursive addition was 'depth == 0' when it
should be 'depth > 0'.

Reviewed-by: Tomas Mraz <tmraz@fedoraproject.org>
(Merged from https://github.com/openssl/openssl/pull/10402)
