Document PUP-4201 JSON logging via --logdest

PUP-4201 added support for specifying --logdest /var/log/somefile.json and getting a (partial) JSON array of log message objects. This wasn't documented.

Docs for --logdest are in several files in lib/puppet/application/<NAME>.rb. The most important ones are apply, agent, and master. All three of these, at least, need edits to reflect the new feature.

This feature is quiet enough and odd enough that I believe updating the description of --logdest is the only documentation we need.

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