NAPI: kconfig prompt and deleted doc file

- make the kconfig NAPI option prompt consistent across all net drivers
  (other than EXPERIMENTAL; can it now be removed also, or is the new
  napi_struct implementation now EXPERIMENTAL ?)
- remove comment about the now-deleted NAPI_HOWTO.txt file
- clean up typos in Tulip NAPI & Interrupt Mitigation

Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>