doc/_ext: ignore desc if it is unavailable

doc/_ext: ignore desc if it is unavailable

there is chance that we don't have desc, desc_long or fmt_desc, in that
case, we should just skip desc before checking its length. so, just use
'if desc' which is able to check for None or empty string.

Signed-off-by: Kefu Chai <kchai@redhat.com>