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unrelated to the ticket, but this would be a good opportunity to fix these
trivial things.

This fixes a build failure on the build bots.

Conflicts:
	src/lib/dhcp/iface_mgr.cc

* Newlines have been added to the default values where it returns
  "Unknown" to preserve formatting.

* If a command fails, a failure message is returned instead of
  "Unknown".

changed or added some docbook formatting tags

slight grammar or punctuation changes

some clarifications or rewording; minor explanations.

removed some superfluous content or examples or simplified.

This was not reviewed.

- style fix in return value
- updated header documentation
- added common label count checks to tests