net: filter: mention eBPF terminology as well

Since the term eBPF is used anyway on mailing list discussions, lets
also document that in the main BPF documentation file and replace a
couple of occurrences with eBPF terminology to be more clear.

Signed-off-by: Alexei Starovoitov <ast@plumgrid.com>
Signed-off-by: Daniel Borkmann <dborkman@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>