doc: packet_mmap: update doc to implementation status

This improves the packet_mmap.txt document in the following ways:

 * Add initial information about different TPACKET versions
 * Add initial information about packet fanout
 * Add pointer to BPF document (since this also could be of interest)
 * 'Fix' minor, rather cosmetic things

Information partially taken from related commit messages.

Reported-by: Ronny Meeus <ronny.meeus@gmail.com>
Signed-off-by: Daniel Borkmann <daniel.borkmann@tik.ee.ethz.ch>
Cc: Ulisses Alonso Camaró <uaca@alumni.uv.es>
Cc: Johann Baudy <johann.baudy@gnu-log.net>
Signed-off-by: David S. Miller <davem@davemloft.net>