[css-text-3] Add note about hyphenating mixed case words

Closes #3927

Author: frivoal

css-text-3/Overview.bs

@@ -2830,6 +2830,13 @@ Line Breaking Details</h3>
           <wpt pathprefix="/css/vendor-imports/mozilla/mozilla-central-reftests/text3/">
             hyphenation-control-1.html
           </wpt>
+
+          Note: In some languages
+          (such as English but not German),
+          it may appropriate to avoid having hyphenation opportunities in mixed case words,
+          as those may indicate proper nouns.
+          This type of heuristic is however not mandated by this specification,
+          as is up to the User Agent and its language-specific hyphenation resource.
     </dl>
 
     <p>Correct automatic hyphenation requires a hyphenation resource