Document that std::initializer_list is not always available.

Differential Revision: http://llvm-reviews.chandlerc.com/D2923

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@202750 91177308-0d34-0410-b5e6-96231b3b80d8

docs/CodingStandards.rst

@@ -153,6 +153,9 @@ being aware of:
 * While most of the atomics library is well implemented, the fences are
   missing. Fortunately, they are rarely needed.
 * The locale support is incomplete.
+* ``std::initializer_list`` (and the constructors and functions that take it as
+  an argument) are not always available, so you cannot (for example) initialize
+  a ``std::vector`` with a braced initializer list.
 
 Other than these areas you should assume the standard library is available and
 working as expected until some build bot tells you otherwise. If you're in an