doc: refine ethernet and VLAN flow rule items

doc: refine ethernet and VLAN flow rule items

Specified pattern may be translated in different manner.
For example the pattern "eth / ipv4" can be translated to match
untagged packets only, since the pattern doesn't specify a VLAN item.
It can also be translated to match both tagged and untagged packets,
for the same reason.
This patch updates the rte_flow documentation to clearly specify the
required pattern to use.
For example:
To match tagged ipv4 packets, the pattern "eth / vlan / ipv4 / end"
should be used.
To match untagged ipv4 packets, the pattern "eth / ipv4 / end"
should be used.
To match all IPV4 packets, both tagged and untagged, need to apply
two rules with the patterns above.
To match both tagged and untagged packets of any type, the pattern
"eth / end" should be used.

Signed-off-by: Dekel Peled <dekelp@mellanox.com>
Acked-by: Andrew Rybchenko <arybchenko@solarflare.com>
Acked-by: Ori Kam <orika@mellanox.com>