[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/ResponseError.php

@@ -11,26 +11,46 @@
 
 namespace Predis;
 
+/**
+ * Represents an error returned by Redis (-ERR replies) during the execution
+ * of a command on the server.
+ *
+ * @author Daniele Alessandri <suppakilla@gmail.com>
+ */
 class ResponseError implements IRedisServerError
 {
     private $_message;
 
+    /**
+     * @param string $message Error message returned by Redis
+     */
     public function __construct($message)
     {
         $this->_message = $message;
     }
 
+    /**
+     * {@inheritdoc}
+     */
     public function getMessage()
     {
         return $this->_message;
     }
 
+    /**
+     * {@inheritdoc}
+     */
     public function getErrorType()
     {
         list($errorType, ) = explode(' ', $this->getMessage(), 2);
         return $errorType;
     }
 
+    /**
+     * Converts the object to its string representation.
+     * 
+     * @return string
+     */
     public function __toString()
     {
         return $this->getMessage();