[namespace.def.general] Clarify inline namespaces

Highlight 'innermost enclosing' namespace, and remove
unneeded mechanism of inserting a using-directive.

Author: jensmaurer

source/declarations.tex

@@ -7218,12 +7218,10 @@
 
 \pnum
 Members of an inline namespace can be used in most respects as though they were members
-of the enclosing namespace. Specifically, the inline namespace and its enclosing
+of the innermost enclosing namespace. Specifically, the inline namespace and its enclosing
 namespace are both added to the set of associated namespaces used in
-argument-dependent lookup\iref{basic.lookup.argdep} whenever one of them is,
-and a \grammarterm{using-directive}\iref{namespace.udir} that names the inline
-namespace is implicitly inserted into the enclosing namespace as for an unnamed
-namespace\iref{namespace.unnamed}. Furthermore, each
+argument-dependent lookup\iref{basic.lookup.argdep} whenever one of them is.
+Furthermore, each
 member of the inline namespace can subsequently be partially
 specialized\iref{temp.spec.partial}, explicitly
 instantiated\iref{temp.explicit}, or explicitly specialized\iref{temp.expl.spec} as