bpf, docs: Shift operations are defined to use a mask

Update the documentation regarding shift operations to explain the
use of a mask, since otherwise shifting by a value out of range
(like negative) is undefined.

Signed-off-by: Dave Thaler <dthaler@microsoft.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/bpf/20230509180845.1236-1-dthaler1968@googlemail.com