[phpdoc] Add API documentation.

This is a starting point to add a documentation of the whole set of APIs and
classes of Predis. The next step will be to actually improve and extend it.

lib/Predis/ClientException.php

@@ -11,6 +11,11 @@
 
 namespace Predis;
 
+/**
+ * Exception class that identifies client-side errors.
+ *
+ * @author Daniele Alessandri <suppakilla@gmail.com>
+ */
 class ClientException extends PredisException
 {
 }