DOC: add explanation to API docs
API Reference
=============
+This is the class and function reference of Altair, and the following content
+is generated automatically from the code documentation strings.
+Please refer to the `full user guide <http://altair-viz.github.io>`_ for
+further details, as this low-level documentation may not be enough to give
+full guidelines on their use.
+
Top-Level Objects
-----------------
