[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/ServerException.php

@@ -11,14 +11,29 @@
 
 namespace Predis;
 
+/**
+ * Exception class that identifies server-side Redis errors.
+ *
+ * @author Daniele Alessandri <suppakilla@gmail.com>
+ */
 class ServerException extends PredisException implements IRedisServerError
 {
+    /**
+     * Gets the type of the error returned by Redis.
+     *
+     * @return string
+     */
     public function getErrorType()
     {
         list($errorType, ) = explode(' ', $this->getMessage(), 2);
         return $errorType;
     }
 
+    /**
+     * Converts the exception to an instance of ResponseError.
+     *
+     * @return ResponseError
+     */
     public function toResponseError()
     {
         return new ResponseError($this->getMessage());