Explain that '**kw' arg to 'add_value' is only useful for string values.

Addresses:
https://github.com/GoogleCloudPlatform/gcloud-python/pull/1173/files#r41574050

Author: tseaver

gcloud/search/document.py

@@ -119,7 +119,8 @@ def add_value(self, value, **kw):
         :param value: the field value to add.
 
         :param kw: extra keyword arguments to be passed to the value instance
-                   constructor.
+                   constructor.  Currently, only :class:`StringValue`
+                   expects / honors additional parameters.
 
         :raises: ValueError if unable to match the type of ``value``.
         """