futex: Correct the kernedoc return value for futex_wait_setup().

futex: Correct the kernedoc return value for futex_wait_setup().

The kerneldoc for futex_wait_setup() states it can return "0" or "<1".
This isn't true because the error case is "<0" not less than 1.

Document that <0 is returned on error. Drop the possible return values
and state possible reasons.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: André Almeida <andrealmeid@igalia.com>
Link: https://lore.kernel.org/r/20250517151455.1065363-6-bigeasy@linutronix.de