Text suggested by Daniel Berlin. While it is likely to be exactly what
the advisory committee would do anyway, codifying it does no harm and
helps reassure people that rare does not mean arbitrary.
Differential Revision: http://reviews.llvm.org/D21981
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@274659 91177308-0d34-0410-b5e6-96231b3b80d8