Update admission_control.md
Update admission_control.md

I tested out a Limit Ranger, and it seems like the admission happens *before* Validation. Please correct me if I'm wrong though, I didn't look at the code in detail. In any case, I think it makes sense for admission to happen before validation because code in admission can change containers.

By the way I think it's pretty hard to find flows like this in the code, so it's useful if we add links to code in the design docs (for prospective developers) :)

kubernetes/community,kubernetes/community,bgrant0607/community,pwittrock/community,pwittrock/community,kubernetes/community,bgrant0607/community