Add docs for the bigquery dataset delete_contents_on_destroy field (h…

…ashicorp#416)

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/cc @rileykarson

website/docs/r/bigquery_dataset.html.markdown

@@ -52,6 +52,10 @@ The following arguments are supported:
 
 * `description` - (Optional) A user-friendly description of the dataset.
 
+* `delete_contents_on_destroy` - (Optional) If set to `true`, delete all the
+    tables in the dataset when destroying the resource; otherwise, destroying
+    the resource will fail if tables are present.
+
 * `location` - (Optional) The geographic location where the dataset should reside.
     See [official docs](https://cloud.google.com/bigquery/docs/dataset-locations).