[lex.string] Mark "narrow string literal" as a definition (#6533)

Author: eisenwave

source/lex.tex

@@ -1880,10 +1880,9 @@
 \end{example}
 
 \pnum
-\indextext{literal!string!narrow}%
 \indextext{literal!narrow-character}%
 Ordinary string literals and UTF-8 string literals are
-also referred to as narrow string literals.
+also referred to as \defnx{narrow string literals}{literal!string!narrow}.
 
 \pnum
 \indextext{concatenation!string}%