Update documentation

Author: allenc3

google-cloud-bigquerystorage/src/main/java/com/google/cloud/bigquery/storage/v1alpha2/BQTableSchemaToProtoDescriptor.java

@@ -74,6 +74,7 @@ public static Descriptor ConvertBQTableSchemaToProtoDescriptor(Table.TableSchema
    * @param BQTableSchema
    * @param scope Keeps track of current scope to prevent repeated naming while constructing
    *     descriptor.
+   * @param dependencyMap Stores already constructed descriptors to prevent reconstruction
    * @throws Descriptors.DescriptorValidationException
    */
   private static Descriptor ConvertBQTableSchemaToProtoDescriptorImpl(