Update documentation for java build on various platforms

[whd]

From https://github.com/mozilla/bigquery-backfill/pull/20, though there's going to be another TODO to switch to using the pre-built flex templates instead of rebuilding main in order to do backfills. However, it's a fairly common case that backfills may need to be built from branch for e.g. decoder tweaks so being able to build and run new code is still a good thing to have.