[lex.string] Clarify size of string-literal

Author: jensmaurer

source/lex.tex

@@ -1705,7 +1705,9 @@
 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}
-where $n$ is the number of encoded code units as described below.
+where $n$ is the number of encoded code units
+in the result of an evaluation of the \grammarterm{string-literal}
+were it to be evaluated (see below).
 
 \begin{floattable}{String literals}{lex.string.literal}
 {llp{2.6cm}p{2.3cm}p{4.7cm}}