ethdev: clarify flow API pattern items and actions

ethdev: clarify flow API pattern items and actions

Although pattern items and actions examples end with "and so on", these
lists include all existing definitions and as a result are updated almost
every time new types are added. This is cumbersome and pointless.

This patch also synchronizes Doxygen and external API documentation wording
with a slight clarification regarding meta pattern items.

No fundamental API change.

Signed-off-by: Adrien Mazarguil <adrien.mazarguil@6wind.com>
Acked-by: Andrew Rybchenko <arybchenko@solarflare.com>