sched_ext: Document the ops compat strategy in compat.h/compat.bpf.h

sched_ext: Document the ops compat strategy in compat.h/compat.bpf.h

The comments around SCX_OPS_DEFINE() and SCX_OPS_OPEN() were vague about
how backward compatibility actually works. Expand them to describe the two
mechanisms: load-time BTF fix-up for additive changes, and multi-variant
struct_ops for incompatible ones.

Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Andrea Righi <arighi@nvidia.com>
Acked-by: Cheng-Yang Chou <yphbchou0911@gmail.com>