Network interface synchronization docs

I just updated Documentation/networking/netdevices.txt locally to the attached file, which describes the synchronization rules for net drivers in the 2.4.x and 2.5.x kernels.  (Astute readers will notice a striking similarity to 2.2.x locking rules as well)

Comments and corrections to the attached doc requested.  I realize it's not much right now, but this is core info I want to make sure is correct, and that everyone agrees on.

Thanks to Robert Olsson for a pointer about dev->poll().

Future notes:  this doc will hopefully expand in time to describe not only the netdevice API but also standard ethernet and net driver practices.  Patches welcome!

	Jeff