docs: update clustering field docstrings (#774)

Author: ryanyuan

google-cloud-bigquery/src/main/java/com/google/cloud/bigquery/StandardTableDefinition.java

@@ -157,7 +157,7 @@ public abstract static class Builder
 
     /**
      * Set the clustering configuration for the table. If not set, the table is not clustered.
-     * Clustering is only available for partitioned tables.
+     * BigQuery supports clustering for both partitioned and non-partitioned tables.
      */
     public abstract Builder setClustering(Clustering clustering);
 

samples/snippets/src/main/java/com/example/bigquery/CreateClusteredTable.java

@@ -55,7 +55,7 @@ public static void createClusteredTable(
 
       TimePartitioning partitioning = TimePartitioning.of(TimePartitioning.Type.DAY);
       // Clustering fields will be consisted of fields mentioned in the schema.
-      // As of now, another condition is that the table should be partitioned.
+      // BigQuery supports clustering for both partitioned and non-partitioned tables.
       Clustering clustering = Clustering.newBuilder().setFields(clusteringFields).build();
 
       StandardTableDefinition tableDefinition =

samples/snippets/src/main/java/com/example/bigquery/LoadTableClustered.java

@@ -65,7 +65,7 @@ public static void loadTableClustered(
 
       TimePartitioning partitioning = TimePartitioning.of(TimePartitioning.Type.DAY);
       // Clustering fields will be consisted of fields mentioned in the schema.
-      // As of now, another condition is that the table should be partitioned.
+      // BigQuery supports clustering for both partitioned and non-partitioned tables.
       Clustering clustering = Clustering.newBuilder().setFields(clusteringFields).build();
 
       LoadJobConfiguration loadJobConfig =