docs: clarify %%bigquery` magics and fix broken link (#508)

Author: tswast

docs/usage/index.rst

@@ -29,7 +29,7 @@ Integrations with Other Libraries
 
  pandas
 
-See also, the :mod:`google.cloud.bigquery.magics` module for integrations
-with Jupyter.
+See also, the :mod:`google.cloud.bigquery.magics.magics` module for
+integrations with Jupyter.
 
 

google/cloud/bigquery/magics/magics.py

@@ -14,6 +14,15 @@
 
 """IPython Magics
 
+To use these magics, you must first register them. Run the ``%load_ext`` magic
+in a Jupyter notebook cell.
+
+.. code::
+
+ %load_ext google.cloud.bigquery
+
+This makes the ``%%bigquery`` magic available.
+
 .. function:: %%bigquery
 
  IPython cell magic to run a query and display the result as a DataFrame