[libcxx] Mention restriction on inline namespaces in LIBCXX_ABI_NAMESPACE docs

I also kept the original "vague" documentation that saying that users are
responsible for not breaking us. This doesn't mean anything because there's
no way they can actually enforce that unless we restrict ourselves to a
specific naming scheme, but I left the documentation because it acts as a
good warning and gives us more leeway.

llvm-svn: 347052