LangRef: add a note about the mangling-suppressing \01 prefix

Someone asked about this on IRC the other day, and I couldn't
find the magic prefix documented anywhere.

Differential Revision: http://reviews.llvm.org/D4728

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@214329 91177308-0d34-0410-b5e6-96231b3b80d8