[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/Distribution/INodeKeyGenerator.php

@@ -11,7 +11,19 @@
 
 namespace Predis\Distribution;
 
+/**
+ * A generator of node keys implements the logic used to calculate the hash of
+ * a key to distribute the respective operations among nodes.
+ *
+ * @author Daniele Alessandri <suppakilla@gmail.com>
+ */
 interface INodeKeyGenerator
 {
+    /**
+     * Generates an hash that is used by the distributor algorithm
+     *
+     * @param string $value Value used to generate the hash.
+     * @return int
+     */
     public function generateKey($value);
 }