Update pandas.read_gbq docs to point to pandas-gbq (#18548)

(cherry picked from commit 5cd5e3b)

Author: tswast

doc/source/install.rst

@@ -258,7 +258,8 @@ Optional Dependencies
   <http://www.vergenet.net/~conrad/software/xsel/>`__, or `xclip
   <https://github.com/astrand/xclip/>`__: necessary to use
   :func:`~pandas.read_clipboard`. Most package managers on Linux distributions will have ``xclip`` and/or ``xsel`` immediately available for installation.
-* For Google BigQuery I/O - see `here <https://pandas-gbq.readthedocs.io/en/latest/install.html#dependencies>`__
+* `pandas-gbq <https://pandas-gbq.readthedocs.io/en/latest/install.html#dependencies>`__: for Google BigQuery I/O.
+
 
 * `Backports.lzma <https://pypi.python.org/pypi/backports.lzma/>`__: Only for Python 2, for writing to and/or reading from an xz compressed DataFrame in CSV; Python 3 support is built into the standard library.
 * One of the following combinations of libraries is needed to use the

pandas/io/gbq.py

@@ -29,9 +29,8 @@ def read_gbq(query, project_id=None, index_col=None, col_order=None,
     The main method a user calls to execute a Query in Google BigQuery
     and read results into a pandas DataFrame.
 
-    Google BigQuery API Client Library v2 for Python is used.
-    Documentation is available `here
-    <https://developers.google.com/api-client-library/python/apis/bigquery/v2>`__
+    This function requires the `pandas-gbq package
+    <https://pandas-gbq.readthedocs.io>`__.
 
     Authentication to the Google BigQuery service is via OAuth 2.0.
 
@@ -70,7 +69,7 @@ def read_gbq(query, project_id=None, index_col=None, col_order=None,
 
     dialect : {'legacy', 'standard'}, default 'legacy'
         'legacy' : Use BigQuery's legacy SQL dialect.
-        'standard' : Use BigQuery's standard SQL (beta), which is
+        'standard' : Use BigQuery's standard SQL, which is
         compliant with the SQL 2011 standard. For more information
         see `BigQuery SQL Reference
         <https://cloud.google.com/bigquery/sql-reference/>`__