Document what's experimental about __attribute__((enable_if)). PR20392


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@215806 91177308-0d34-0410-b5e6-96231b3b80d8
Try indenting to put all the text inside the block.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@215807 91177308-0d34-0410-b5e6-96231b3b80d8
Clarify.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@215808 91177308-0d34-0410-b5e6-96231b3b80d8