[css-text] Clarify effect of line-break:anywhere on white space

Closes w3c#3561

Author: frivoal

css-text-3/Overview.bs

@@ -813,7 +813,7 @@ Text Processing</h3>
       </wpt>
   </dl>
 
-  <p>White space that was not removed or collapsed due to white space processing
+  <p>[=White space=] that was not removed or collapsed due to white space processing
     is called <dfn local-lt="preserved">preserved white space</dfn>.
     In general, <a>preserved white space</a> is measured when calculating
     the intrinsic sizes ([=min-content size=] and [=max-content size=]) of text;
@@ -2408,7 +2408,7 @@ Line Breaking Details</h3>
         rules.</dd>
       <dt><dfn>anywhere</dfn></dt>
       <dd>There is a <a>soft wrap opportunity</a> around every <a>typographic character unit</a>,
-      including around any punctuation character or preserved spaces,
+      including around any punctuation character or [=preserved white spaces=],
       or in the middle of words,
       disregarding any prohibition against line breaks,
       even those introduced by characters with the GL, WJ, or ZWJ character class (see [[UAX14]])
@@ -2422,6 +2422,20 @@ Line Breaking Details</h3>
       </wpt>
 
       Note: This value triggers the line breaking rules typically seen in terminals.</dd>
+
+      <div class=note>
+        Note: ''line-break/anywhere'' only allows [=preserved white spaces=] at the end of the line
+        to be wrapped to the next line when 'white-space' is set to ''white-space/break-spaces'',
+        because in other cases:
+        * [=preserved white space=] at the end/start of the line is discarded (''white-space/normal'', ''white-space/pre-line'')
+        * wrapping is forbidden altoghether (''nowrap'', ''pre'')
+        * the spaces [=hang=] (''pre-wrap'').
+
+        When it does have an effect on [=preserved white space=],
+        with ''white-space: break-spaces'',
+        it allows breaking before the first space of a sequence,
+        which ''break-spaces'' on its own does not.
+      </div>
     </dl>
 
     <p class="feedback issue">