expand on checkpointing documentation

Documentation on checkpointing (https://pegasus.isi.edu/documentation/user-guide/data-transfers.html?highlight=checkpoint#staging-of-job-checkpoint-files) is to be updated based on content in https://gist.github.com/ryantanaka/1249fe3ef90df100a80a2724bc29624f.

Also add that maxwalltime may not be needed if you give a nonzero return code.