[defns.character.container] Improve note to entry (#6644)

Author: jwakely

source/intro.tex

@@ -183,8 +183,9 @@
 represent a character
 
 \begin{defnote}
-It is used for one of the template parameters of the string,
-iostream, and regular expression class templates.
+It is used for one of the template parameters of \tcode{char_traits}
+and the class templates which use that,
+such as the string, iostream, and regular expression class templates.
 \end{defnote}
 
 \definition{collating element}{defns.regex.collating.element}