Fix comments in mixer/v1/config/cfg.proto

[douglas-reid]

The examples and comments for messages like Instance and Handler are no longer correct or valid.

[guptasu]

Are they not valid because they in not in CR form ? Or there is something else missing ?

[geeknoid]

Can this be closed? Are the examples correct at this point?

[douglas-reid]

I think most of the problematic bits can be removed as part of the removal of all the long-deprecated bits.

I also think it probably makes sense to update the examples for the bits that remain to match the CR form that we expect operators to interact with.