BigQueryIO WRITE_TRUNCATE documentation is out of date

Apache Beam 2.5 documentation ([https://beam.apache.org/documentation/sdks/javadoc/2.5.0/](https://beam.apache.org/documentation/sdks/javadoc/2.5.0/)), states that
 
"WRITE_TRUNCATE
public static final BigQueryIO.Write.WriteDisposition WRITE_TRUNCATE
Specifies that write should replace a table.
The replacement *may occur in multiple steps* - for instance by first removing the existing table, then creating a replacement, then filling it in. *This is not an atomic operation*, and external programs may see the table in any of these intermediate steps."
 
However WRITE_TRUNCATE is an atomic BigQuery operation. The documentation needs to be updated.

Imported from Jira [BEAM-4789](https://issues.apache.org/jira/browse/BEAM-4789). Original Jira may contain additional context.
Reported by: altay.

Start with the Apache Beam 2.5 Javadoc entry for BigQueryIO.Write.WriteDisposition and inspect the WRITE_TRUNCATE documentation. Update the statement that replacement may occur in multiple steps so it reflects atomic BigQuery behavior, then verify the Javadoc contains the corrected wording.

Written by the indexing model from the issue text.