updating spec and test for check_positive_ordered

#### Summary:

Updates documentation for `check_positive_ordered()`, which was incorrectly documented, but implemented correctly. Added unit tests.

#### Intended Effect:

No change to code.

#### How to Verify:

Run the new unit tests.

#### Side Effects:

No.

#### Documentation:

I checked the latex, but didn't see any documentation stating that duplicates weren't allowed or unallowed, so I left it alone. The developer facing documentation was updated.

#### Reviewer Suggestions:

Anyone.