Fix CLI description printout

[opotowsky]

What is the change?

Splits what prints of the description for the ARMI CLI at the impl tag.

Why is the change being made?

Because the full docstring was being used, the impl tag was being printed when you run armi -h:

This PR fixes that to:

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Checklist

• This PR has only one purpose or idea.
• Tests have been added/updated to verify that the new/changed code works.

• The code style follows good practices.
• The commit message(s) follow good practices.

• The release notes (location doc/release/0.X.rst) are up-to-date with any important changes.
• The documentation is still up-to-date in the doc folder.
• If any requirements were affected, mention it in the release notes.
• The dependencies are still up-to-date in pyproject.toml.

[john-science]

Approved!

Thanks!