[format.string.std] Clarify that "extended grapheme clusters" is defined

by UAX #29, but "estimated widths" is not.

Author: zygoloid

source/utilities.tex

@@ -19515,8 +19515,8 @@
 For a string in a Unicode encoding,
 implementations should estimate the width of a string
 as the sum of estimated widths of
-the first code points in its extended grapheme clusters
-as defined by UAX \#29.
+the first code points in its extended grapheme clusters.
+The extended grapheme clusters of a string are defined by UAX \#29.
 The estimated width of the following code points is 2:
 \begin{itemize}
 \item \tcode{U+1100-U+115F}