[lld][LayoutPass] Add comment on _followOn{Nexts,Roots}.

Reviewers: shankarke

CC: llvm-commits

Differential Revision: http://llvm-reviews.chandlerc.com/D847

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@182595 91177308-0d34-0410-b5e6-96231b3b80d8