* doc/mk-extension.zip.md: mention external facts, reword some bits

This change depends on the pull request
https://github.com/puppetlabs/razor-el-mk/pull/23 for the microkernel
Command's `to_s` shouldn't be help text

In certain cases where spec tests fail, the entire help text was getting spat out. This reverts back to a more sensible output, e.g. "Razor::Command::CreatePolicy".

Fixes https://tickets.puppetlabs.com/browse/RAZOR-240