CWG3090 Internal linkage from header units

Fixes NB US 43-080 (C++26 CD).

Author: eisenwave

source/modules.tex

@@ -334,8 +334,9 @@
 
 \pnum
 \begin{note}
-Names introduced by exported declarations
-have either external linkage or no linkage; see \ref{basic.link}.
+Names introduced by exported declarations never have module linkage.
+They have external linkage, no linkage, or
+(in the case of header units) internal linkage; see \ref{basic.link}.
 Namespace-scope declarations exported by a module can be found by name lookup
 in any translation unit importing that module\iref{basic.lookup}.
 Class and enumeration member names can be found by name lookup in any