Add longer descriptions to impls in flags

[keckler]

What is the change?

Just adds descriptions to the implementation tags in the flags module.

Why is the change being made?

Quality.

---

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]

The unit tests for this PR are throwing the error:

  File "/home/runner/work/armi/armi/armi/reactor/flags.py", line 253, in fromStringIgnoreErrors
    return __fromStringGeneral(cls, typeSpec, updateMethodIgnoreErrors)
NameError: name '_Flags__fromStringGeneral' is not defined

[linconnor]

The added impls info looks good.