[lex.string] Add xref to lex.charset, where encodings are defined

Author: tkoeppe

source/lex.tex

@@ -1902,7 +1902,7 @@
 \indextext{type!\idxcode{char32_t}}%
 The kind of a \grammarterm{string-literal},
 its type, and
-its associated character encoding
+its associated character encoding\iref{lex.charset}
 are determined by its encoding prefix and sequence of
 \grammarterm{s-char}s or \grammarterm{r-char}s
 as defined by \tref{lex.string.literal}