[mlir] clarify LangRef wording around control flow in regions
It was unclear what "exiting a region" meant in the existing formulation.
Phrase it in terms of control flow transfer to the operation enclosing the
region.
Discussion: https://groups.google.com/a/tensorflow.org/d/msg/mlir/73d2O8gjTuA/xVj1KoCTBAAJ
