Document X509_get_default_cert_dir_env(3)

Document X509_get_default_cert_dir_env(3)
and X509_get_default_cert_file_env(3).

LibreSSL itself does not call getenv(3), but a few application programs
including epic5, fetchmail, fossil, slic3r call these functions, so in
case programmers find them in existing code, telling them what they do
seems useful.