docs: net: netdevices: small fixes and clarifications

docs: net: netdevices: small fixes and clarifications

A handful of unrelated nits:

 - free_netdevice() does not exist; replace two stray references
   with free_netdev().
 - The simple-driver probe example fell through into err_undo after
   register_netdev() success; add return 0 for clarity.
 - Clarify the netdev_priv() paragraph: "(netdev_priv())" was easy
   to misread as the thing that needs explicit freeing; spell out
   that it refers to extra pointers stored in the device private
   struct.
 - ndo_setup_tc synchronization note: TC_SETUP_BLOCK / TC_SETUP_FT
   actually run under block->cb_lock, not "NFT locks", and rtnl_lock
   may or may not be held depending on path.
 - ->lltx guidance reads as very outdated, it's not really deprecated.
   I suspect people may have been trying to use it for HW drivers
   in the past but I can't think of such a case in the last decade.

Acked-by: Stanislav Fomichev <sdf@fomichev.me>
Link: https://patch.msgid.link/20260526160151.2793354-2-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>