doc: add linux/doc.go
doc: update Setup section
doc: update doc comments
unexport: NameScanResponsePacket & ServiceAdvertisingPacket
Shorten AdvertisingOption to Option

Currently, it is exported so that server_linux.go can
use them to override some advertisement param/data.
rework advertiser related code
sync between APSs and server initialization

In some sophiscated usecases, Serve(or AdvertiseAndServe) and
other APIs, such as Advertise, might be called by different places.