Comment parsing: followup to r184610: allow multiple \returns

Remove unneeded member in CommentSema, add a test for the XML schema (the
schema already allowed multiple paragraphs in <ResultDiscussion>, but there
were no tests for that), fix HTML generation (it is not allowed to have <p>
inside <dl>).


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@184652 91177308-0d34-0410-b5e6-96231b3b80d8