Code review comment for ~ffledgling/cloud-init:doc-fix

In addition to tests, there are tests (namely: tests/data/merge_sources/source8-1.yaml, tests/data/merge_sources/expected8.yaml) and some code (config?) files (namely: cloudinit/config/cc_mounts.py) that use nobootwait as well, but I haven't modified them because I'm not entirely clear about the scope of those changes. If those need to be changed as well, please let me know.