futex2: Documentation: Document sys_futex_waitv() uAPI

Create userspace documentation for futex_waitv() syscall, detailing how
the arguments are used.

Signed-off-by: André Almeida <andrealmeid@collabora.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/r/20210923171111.300673-23-andrealmeid@collabora.com