[css-text-4] Use more precise terminology

(relates to w3c#4284)

Author: frivoal

css-text-4/Overview.bs

@@ -145,12 +145,12 @@ Detecting Word Boundaries: the 'word-boundary-detection' property</h4>
 			the User Agent should use this information
 			to tailor its analysis.
 
-			In order to avoid overflow,
+			In order to avoid unexpected overflow,
 			if the User Agent is unable to perform this analysis
 			for any subset of the characters with class SA--
 			for example due to lacking a dictionary for certain languages--
 			there must be a [=soft wrap opportunity=]
-			between pairs of [=typographic character units=] in that subset.
+			between pairs of [=typographic letter units=] in that subset.
 
 			Note: This [=soft wrap opportunity=] is not
 			a [=virtual word boundary=],