docs: update clustering field docstrings (#286)

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Fixes #285  🦕

google/cloud/bigquery/job.py

@@ -1073,8 +1073,8 @@ def clustering_fields(self):
 
         .. note::
 
-           As of 2018-06-29, clustering fields cannot be set on a table
-           which does not also have time partioning defined.
+           BigQuery supports clustering for both partitioned and
+           non-partitioned tables.
         """
         prop = self._get_sub_prop("clustering")
         if prop is not None:
@@ -2554,8 +2554,8 @@ def clustering_fields(self):
 
         .. note::
 
-           As of 2018-06-29, clustering fields cannot be set on a table
-           which does not also have time partioning defined.
+           BigQuery supports clustering for both partitioned and
+           non-partitioned tables.
         """
         prop = self._get_sub_prop("clustering")
         if prop is not None:

google/cloud/bigquery/table.py

@@ -625,8 +625,8 @@ def clustering_fields(self):
 
         .. note::
 
-           As of 2018-06-29, clustering fields cannot be set on a table
-           which does not also have time partioning defined.
+           BigQuery supports clustering for both partitioned and
+           non-partitioned tables.
         """
         prop = self._properties.get("clustering")
         if prop is not None:
@@ -1030,8 +1030,8 @@ def clustering_fields(self):
 
         .. note::
 
-           As of 2018-06-29, clustering fields cannot be set on a table
-           which does not also have time partioning defined.
+           BigQuery supports clustering for both partitioned and
+           non-partitioned tables.
         """
         prop = self._properties.get("clustering")
         if prop is not None: