chore(java-bigquerydatapolicy): update api_description for bigquerydatapolicy (#13010)

Add api_description for bigquerydatapolicy according to
documentation.summary
[here](https://github.com/googleapis/googleapis/blob/68f8f68867b0cfd4e6eacaced05494b85afbfcf0/google/cloud/bigquery/datapolicies/v2/bigquerydatapolicy_v2.yaml#L10).

This should update .repo-metadata.json and README.md via generate.

Fix googleapis/librarian#5562

---------

Co-authored-by: cloud-java-bot <cloud-java-bot@google.com>

Author: zhumin8

generation_config.yaml

@@ -391,7 +391,7 @@ libraries:
 - api_shortname: bigquerydatapolicy
   name_pretty: BigQuery DataPolicy API
   product_documentation: https://cloud.google.com/bigquery/docs/reference/datapolicy/
-  api_description: ''
+  api_description: Allows users to manage BigQuery data policies.
   GAPICs:
   - proto_path: google/cloud/bigquery/datapolicies/v1
   - proto_path: google/cloud/bigquery/datapolicies/v1beta1

java-bigquerydatapolicy/.repo-metadata.json

@@ -2,7 +2,7 @@
   "api_shortname": "bigquerydatapolicy",
   "name_pretty": "BigQuery DataPolicy API",
   "product_documentation": "https://cloud.google.com/bigquery/docs/reference/datapolicy/",
-  "api_description": "",
+  "api_description": "Allows users to manage BigQuery data policies.",
   "client_documentation": "https://cloud.google.com/java/docs/reference/google-cloud-bigquerydatapolicy/latest/overview",
   "release_level": "preview",
   "transport": "both",

java-bigquerydatapolicy/README.md

@@ -87,7 +87,7 @@ to add `google-cloud-bigquerydatapolicy` as a dependency in your code.
 ## About BigQuery DataPolicy API
 
 
-[BigQuery DataPolicy API][product-docs] 
+[BigQuery DataPolicy API][product-docs] Allows users to manage BigQuery data policies.
 
 See the [BigQuery DataPolicy API client library docs][javadocs] to learn how to
 use this BigQuery DataPolicy API Client Library.