bpf: Update bpf_get_smp_processor_id() documentation

BPF programs run with migration disabled regardless of preemption, as
they are protected by migrate_disable(). Update the uapi documentation
accordingly.

Signed-off-by: Matteo Croce <mcroce@microsoft.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/bpf/20210914235400.59427-1-mcroce@linux.microsoft.com