docs: net: bridge: document stp_mode attribute

docs: net: bridge: document stp_mode attribute

Add documentation for the IFLA_BR_STP_MODE bridge attribute in the
"User space STP helper" section of the bridge documentation. Reference
the BR_STP_MODE_* values via kernel-doc and describe the use case for
network namespace environments.

Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Acked-by: Nikolay Aleksandrov <nikolay@nvidia.com>
Signed-off-by: Andy Roulin <aroulin@nvidia.com>
Link: https://patch.msgid.link/20260405205224.3163000-3-aroulin@nvidia.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>