add texnote

doc/environments/groupthm/groupthm.tex

@@ -252,6 +252,15 @@ of the \meta{theorem family}, which then controls the membership in these groups
 
   \end{description}
 
+  \begin{texnote}
+    The \meta{relation} is first stripped,
+    then checked if it matches either \kw{higher} or \kw{lower}
+    and in this case replaced by the corresponding \pkg{lthooks} variant
+    of the relation.
+    The rest is passed as is to \pkg{lthooks} and thus subject to the usual
+    normalization process of \pkg{lthooks}.
+  \end{texnote}
+
 \end{function}
 
 %\section{\LaTeX3 interface}