add example of how to get non-public (proprietary) data

Author: keflavich

docs/alma/alma.rst

@@ -178,6 +178,25 @@ or if you wanted all projects by a given PI:
 
     >>> Alma.query(payload=dict(pi_name='Ginsburg, Adam'))
 
+Querying For Proprietary Data
+-----------------------------
+
+To query for proprietary data (non-public data), you need to first authenticate using the ``login`` method,
+and then set ``public_data=False`` in your query:
+
+.. doctest-skip::
+
+    >>> from astroquery.alma import Alma
+    >>> alma = Alma()
+    >>> # First login to access proprietary data
+    >>> alma.login("username")  # Will prompt for password
+    >>> # Now query including proprietary data
+    >>> proprietary_data = alma.query(payload=dict(project_code='2017.1.01355.L', public_data=False))
+    >>> # Or restrict to only proprietary data
+    >>> only_proprietary = alma.query_region('W51', radius=25*u.arcmin, public_data=False)
+
+When you set ``public_data=False``, the query will include only proprietary data by adding a condition equivalent to ``data_rights='Proprietary'`` in the underlying SQL query.
+
 The ``query_sia`` method offers another way to query ALMA using the IVOA SIA
 subset of keywords returning results in 'ObsCore' format.  For example,
 to query for all images that have ``'XX'`` polarization (note that this query is too large