Fix Yac::__construct documentation. (#570)

Add missing errors section

Author: thinkverse

reference/yac/yac/construct.xml

@@ -44,7 +44,14 @@
  </refsect1>
  -->
 
-
+ <refsect1 role="errors">
+  &reftitle.errors;
+  <para>
+   Throws an <classname>Exception</classname> if Yac is not enabled. Throws 
+   <classname>Exception</classname> if <parameter>prefix</parameter> exceeds 
+   max key length of 48 (<constant>YAC_MAX_KEY_LEN</constant>) bytes.
+  </para>
+ </refsect1>
 
 </refentry>