Touch up [[clang::require_constant_initialization]] docs

* Fix an egregious comma usage.
* Remove the `static` keyword in the example since the variables should have
  external linkage.
* Use C++11 attributes in the example.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@281712 91177308-0d34-0410-b5e6-96231b3b80d8