Add some wording to the coding standards to say how to indent namespaces

Add some wording to the coding standards to say how to indent namespaces
(and to mention namespace ending comments). This is based on a quick
discussion on the developer mailing list where there was essentially no
objections to a simple and consistent rule. This should avoid future
debates about whether or not a namespace is "big enough" to indent. It
also matches clang-format's current behavior with LLVM source code which
hasn't really seen any opposition in code reviews that I spot checked.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@199620 91177308-0d34-0410-b5e6-96231b3b80d8