Clarify that .bit lookups can produce network traffic sometimes

[JeremyRand]

dot-bit/index.md states:

Dot-Bit's digital phonebook doesn't generate any network traffic when you lookup a website address, which improves your privacy.

NS/CNAME/DNAME records will produce network traffic, as will any Namecoin lookups done by a headers-only client like Electrum-NMC. Namecoin probably will generate fewer lookups than the DNS if a non-headers-only client is used, though. Can we clarify this?