[module.interface] Add adjective 'exported' (#5290)

It was clear in context that "the declaration" is exported, but
it is easier to understand if we restate "an exported declaration".

Author: ChuanqiXu9

source/modules.tex

@@ -266,7 +266,7 @@
 \end{example}
 
 \pnum
-If the declaration is a \grammarterm{using-declaration}\iref{namespace.udecl}
+If an exported declaration is a \grammarterm{using-declaration}\iref{namespace.udecl}
 and is not within a header unit,
 all entities to which all of the
 \grammarterm{using-declarator}{s} ultimately refer (if any)