[asan-asm-instrumentation] Added comment describing how asm instrumentation works.

Summary: [asan-asm-instrumentation] Added comment describing how asm instrumentation works.

Reviewers: eugenis

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D5970

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@220670 91177308-0d34-0410-b5e6-96231b3b80d8