docs: net: document new locking reality

Also clarify ndo_get_stats (that read and write paths can run
concurrently) and mention only RCU.

Cc: Saeed Mahameed <saeed@kernel.org>
Signed-off-by: Stanislav Fomichev <sdf@fomichev.me>
Link: https://patch.msgid.link/20250305163732.2766420-14-sdf@fomichev.me


Signed-off-by: Jakub Kicinski <kuba@kernel.org>