[css-text-3] Clarify that if there's a wrap opportunity in the line, content is not allowed to overflow the line.

Author: fantasai

css-text-3/Overview.bs

@@ -809,7 +809,12 @@ Characters and Letters</h3>
     it is a <dfn>soft wrap break</dfn>.
     The process of breaking inline-level content into lines is called <dfn lt="line-breaking-process">line breaking</dfn>.
 
-  <p>Wrapping is only performed at an allowed break point, called a <dfn export>soft wrap opportunity</dfn>.
+  <p>Wrapping is only performed at an allowed break point,
+    called a <dfn export>soft wrap opportunity</dfn>.
+    When wrapping is enabled (see 'white-space'),
+    the UA must minimize the amount of content overflowing a line
+    by wrapping the line at a <a>soft wrap opportunity</a>,
+    if one exists.
 
   <p>In most writing systems,
     in the absence of hyphenation a <a>soft wrap opportunity</a> occurs only at word boundaries.