luci-app-upnp: revise wording to include PCP

- Revise wording, remove redundancies and mention IPv6 limitations
- Mention `UPnP IGD` as `UPnP` is probably not specific enough
- Include current wording in `LUCI_TITLE` for easy finding

Signed-off-by: Sergey Ponomarev <stokito@gmail.com>