BoundedReadFromUnboundedSource should be strongly discouraged

The outputs of reading from a `BoundedReadFromUnboundedSource` are not generally well defined. This should be explicitly noted in the documentation. Potentially, it should be deprecated.

It is also worth considering removing the finalizeCheckpoint calls within the read, to minimally mutate the target source.

Imported from Jira [BEAM-2307](https://issues.apache.org/jira/browse/BEAM-2307). Original Jira may contain additional context.
Reported by: tgroh.

Start by locating BoundedReadFromUnboundedSource and reading its existing documentation. Review the original Jira issue for context, then inspect the finalizeCheckpoint calls mentioned in the issue. Done means the documentation clearly warns about undefined outputs and the project’s decision on deprecation and checkpoint finalization is recorded.

Written by the indexing model from the issue text.