addRequiredTransitive member that Misha added, and explains the whole
concept a lot better. Also, the document used incorrect "subsubsection"
tags instead of "doc_subsubsection" which this fixes.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@12474 91177308-0d34-0410-b5e6-96231b3b80d8