[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

libcxx/docs/BuildingLibcxx.rst

@@ -379,7 +379,9 @@ The following options allow building libc++ for a different ABI version.
 
   .. warning::
     When providing a custom namespace, it's the users responsibility to ensure the name won't cause
-    conflicts with other names defined by libc++, both now and in the future.
+    conflicts with other names defined by libc++, both now and in the future. In particular, inline
+    namespaces of the form ``__[0-9]+`` are strictly reserved by libc++ and may not be used by users.
+    Doing otherwise could cause conflicts and hinder libc++ ABI evolution.
 
 .. option:: LIBCXX_ABI_DEFINES:STRING