HPCC-9629 New language features to support bundles

Add some source documentation

Signed-off-by: Richard Chapman <rchapman@hpccsystems.com>
Merge pull request #4638 from richardkchapman/ecl-bundle-notes

HPCC-9629 New language features to support bundles

Reviewed-by: Gavin Halliday <ghalliday@hpccsystems.com>