Clarify the purpose of external access setting

[cyberb]

External access is used for two things;

1. Allow syncloud.it device DNS to use public IP. By default it uses internal IP to allow DNS based app access inside local network.
2. Maintain UPnP (auto) port mappings.

External access will not change device itself ports, they are always 80 and 443.

Syncloud.it DNS service verifies open ports (internet accessibility) before enabling external access for convenience.

In case of custom domain name external access page should not be used.