Adjust comments regarding non-relocated abbrev offset in debug_info.dwo

I'm not sure the comment in the implementation really adds a lot of
value (it's clear that we emit zero when no symbol is provided, but it
doesn't explain why we would do that). Happy to iterate.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@205386 91177308-0d34-0410-b5e6-96231b3b80d8