docs: net: refresh netdev feature guidance

docs: net: refresh netdev feature guidance

Update netdev feature documentation for current locking rules and
feature semantics. Clarify hw_features updates and netdev_update_features()
locking, keep the NETIF_F_NEVER_CHANGE rule with the VLAN challenged
exception, fix the HSR duplication wording, and document netdev->netmem_tx
as a device flag rather than a feature bit.

Split the list of basic feature sets from the "extra" ones like
vlan_features. A bunch of the newer fields weren't documented and
having them all together would be confusing.

Link: https://patch.msgid.link/20260526160151.2793354-7-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>