[css-text-3] Phrasing tweak

Typographic character units are context dependent and aren't only about
grapheme clusters. Adjust a sentence to take that into account and be a
tad more generic.

Author: frivoal

css-text-3/Overview.bs

@@ -344,6 +344,7 @@ Characters and Letters</h3>
   may be rendered as belonging to either side of the boundary,
   or as some approximation of belonging to both.
   Authors are forewarned that dividing <a>grapheme clusters</a>
+  or ligatures
   by element boundaries may give inconsistent or undesired results.
 
 <h3 id="text-encoding">