[css-break] Clarify that UA may adjust unforced breaks in parallel flows to match, but not unforced breaks.

Author: fantasai

css-break/Overview.bs

@@ -152,6 +152,10 @@ Parallel Fragmentation Flows</h3>
     then a forced break inside the float
     will not affect the content outside the float
     (except insofar as it may increase the height of the float).
+    UAs <em>may</em> (but are not required to)
+    adjust the placement of <a>unforced breaks</a> in parallel <a>formatting contexts</a>
+    to visually balance such side-by-side content,
+    but <em>must not</em> do so to match a <a>forced break</a>.
   </p>
 
   <p>

css-break/Overview.html

@@ -264,7 +264,10 @@ <h3 class="heading settled" data-level="2.1" id="parallel-flows"><span class="se
     For example, if an element is floated,
     then a forced break inside the float
     will not affect the content outside the float
-    (except insofar as it may increase the height of the float). </p>
+    (except insofar as it may increase the height of the float).
+    UAs <em>may</em> (but are not required to)
+    adjust the placement of <a data-link-type="dfn" href="#unforced-break">unforced breaks</a> in parallel <a data-link-type="dfn" href="https://drafts.csswg.org/css-display-3/#formatting-context">formatting contexts</a> to visually balance such side-by-side content,
+    but <em>must not</em> do so to match a <a data-link-type="dfn" href="#forced-break">forced break</a>. </p>
    <p> The following are examples of parallel flows
     whose contents will fragment independently: </p>
    <ul>