doc: netlink: specs: tc: add multi-attr to tc-taprio-sched-entry

Add multi-attr attribute to tc-taprio-sched-entry to specify multiple
entries.

Signed-off-by: Alessandro Marcolini <alessandromarcolini99@gmail.com>
Reviewed-by: Donald Hunter <donald.hunter@gmail.com>
Reviewed-by: Jakub Kicinski <kuba@kernel.org>
Link: https://lore.kernel.org/r/0ba5088ea715103a2bce83b12e2dcbdaa08da6ac.1706962013.git.alessandromarcolini99@gmail.com


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