Add in support for reserved_percentage kwarg and correct documentation.

[qcpeter]

What does this PR do?

What issues does this PR fix or reference?

#54426

Previous Behavior

Misleading documentation.

New Behavior

Adds in reserved_percentage kwarg and updates documentation.

Tests written?

No

Commits signed with GPG?

No

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[Ch3LL]

any chance we can get a test added here to ensure this does not regress? There are currently some unit tests here ./tests/unit/modules/test_extfs.py Other than that this looks great, nice catch :)

[qcpeter]

I'm afraid I'm going away for ten days so will be unable to take a look at this immediately. Presumably you're thinking of some sort of test that passes in the kwargs and tests the flags that are passed the cmd.run mock? The existing tests are pretty minimal.

[waynew]

@qcpeter yeah, that should be sufficient. This doesn't really have a lot of logic, so it shouldn't be too difficult. There may be a test that already exists that could just be extended, but if not, what you described should be 👍

[dwoz]

@qcpeter This needs to be re-opened against the master branch. We are no longer
accepting PRs to 2018.3. You can link back to this PR to preserve the
discussion. Sorry taking so long to get to this and for any confusion.