Added BC break in migration docs

Author: derickr

appendices/migration82/incompatible.xml

@@ -5,6 +5,14 @@
  <sect2 xml:id="migration82.incompatible.date">
   <title>Date</title>
 
+  <para>
+   The DateTime extension has introduced Date extension specific exceptions
+   and errors under the <classname>DateError</classname> and
+   <classname>DateException</classname> hierarchies, instead of warnings and
+   generic exceptions. This improves error handling, at the expense
+   of having having to check for errors and exceptions.
+  </para>
+
   <para>
    <methodname>DateTime::createFromImmutable</methodname> now has a tentative
    return type of <type>static</type>, previously it was <type>DateTime</type>.
@@ -94,7 +102,7 @@
 
   <para>
    <function>var_export</function> no longer omits the leading backslash for exported classes,
-   i.e. these are now fully qualified. 
+   i.e. these are now fully qualified.
   </para>
  </sect2>