[docs] Clarify role of DIExpressions within debug intrinsics

This should make the semantics of DIExpressions within llvm.dbg.{addr,
declare, value} easier to understand.

Differential Revision: https://reviews.llvm.org/D49572

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@338182 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed