Annotate return types for Throwable/Exception/Error as possibly returning null

Author: sgolemon

language/predefined/error/getprevious.xml

@@ -10,7 +10,7 @@
  <refsect1 role="description">
   &reftitle.description;
   <methodsynopsis>
-   <modifier>final</modifier> <modifier>public</modifier> <type>Throwable</type><methodname>Error::getPrevious</methodname>
+   <modifier>final</modifier> <modifier>public</modifier> <type class="union"><type>Throwable</type><type>null</type></type> <methodname>Error::getPrevious</methodname>
    <void />
   </methodsynopsis>
   <para>

language/predefined/exception/getprevious.xml

@@ -10,7 +10,7 @@
  <refsect1 role="description">
   &reftitle.description;
   <methodsynopsis>
-   <modifier>final</modifier> <modifier>public</modifier> <type>Throwable</type><methodname>Exception::getPrevious</methodname>
+   <modifier>final</modifier> <modifier>public</modifier><type class="union"><type>Throwable</type><type>null</type></type><methodname>Exception::getPrevious</methodname>
    <void />
   </methodsynopsis>
   <para>

language/predefined/throwable/getprevious.xml

@@ -10,7 +10,7 @@
  <refsect1 role="description">
   &reftitle.description;
   <methodsynopsis>
-   <modifier>abstract</modifier> <modifier>public</modifier> <type>Throwable</type><methodname>Throwable::getPrevious</methodname>
+   <modifier>abstract</modifier> <modifier>public</modifier> <type class="union"><type>Throwable</type><type>null</type></type> <methodname>Throwable::getPrevious</methodname>
    <void />
   </methodsynopsis>
   <para>