net: netlink: recommend policy range validation

For large ranges (outside of s16) the documentation currently
recommends open-coding the validation, but it's better to use
the NLA_POLICY_FULL_RANGE() or NLA_POLICY_FULL_RANGE_SIGNED()
policy validation instead; recommend that.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Link: https://lore.kernel.org/r/20230127084506.09f280619d64.I5dece85f06efa8ab0f474ca77df9e26d3553d4ab@changeid
Signed-off-by: Jakub Kicinski <kuba@kernel.org>