Improve / Fix documentation examples for Beam 2.4.0 for TextIO and BigQueryIO

The documentation that is part of the code of TextIO has some usage examples.

The description of how to achieve custom naming with the new FileIO in combination with the TextIO could be improved as the code in the "Advanced Features" is not really fully fleshed out and probably not in sync with how it should be done in Beam 2.4.0.

[https://beam.apache.org/documentation/sdks/javadoc/2.4.0/index.html?org/apache/beam/sdk/io/TextIO.html](https://beam.apache.org/documentation/sdks/javadoc/2.4.0/index.html?org/apache/beam/sdk/io/TextIO.html)

See SO Question: 

[https://stackoverflow.com/questions/50930409/how-to-replace-withfilenamepolicy-in-apache-beam-2-4](https://stackoverflow.com/questions/50930409/how-to-replace-withfilenamepolicy-in-apache-beam-2-4)

The second part that could be improved is in the gcp.BigQueryIO class:

[https://github.com/apache/beam/blob/release-2.4.0/sdks/java/io/google-cloud-platform/src/main/java/org/apache/beam/sdk/io/gcp/bigquery/BigQueryIO.java#L217](https://github.com/apache/beam/blob/release-2.4.0/sdks/java/io/google-cloud-platform/src/main/java/org/apache/beam/sdk/io/gcp/bigquery/BigQueryIO.java#L217)

The example is also not fully fleshed out, but most importantly the
`BigQueryIO.write()` should be replaced with `BigQueryIO.write()` as otherwise it won't compile.

Imported from Jira [BEAM-4608](https://issues.apache.org/jira/browse/BEAM-4608). Original Jira may contain additional context.
Reported by: james-woods.