doc: update RSS flow action with best effort

doc: update RSS flow action with best effort

Using the rte_flow action RSS types field,
may result in undefined outcome.

For example selecting both UDP and TCP,
selecting TCP RSS type but the pattern is targeting UDP traffic.
another option is that the PMD doesn't support all requested types.

Until now, it wasn't clear what will happen in such cases.
This commit clarify this issue by stating that the PMD
will work in the best-effort mode, and will fail
in case the requested type is not supported.

Signed-off-by: Ori Kam <orika@nvidia.com>
Signed-off-by: Andrew Rybchenko <arybchenko@solarflare.com>