Docs: Improve polymorphism documentation

[mssalvatore]

What does this PR do?

Improves polymorphism explanation.

PR Checklist

• [ ] Have you added an explanation of what your changes do and why you'd like to include them?
• [ ] Is the TravisCI build passing?
• [ ] Was the CHANGELOG.md updated to reflect the changes?
• [ ] Was the documentation framework updated to reflect the changes?
• [ ] Have you checked that you haven't introduced any duplicate code?

Testing Checklist

• [ ] Added relevant unit tests?
• [ ] Do all unit tests pass?
• [ ] Do all end-to-end tests pass?
• [ ] Any other testing performed?

  Tested by {Running the Monkey locally with relevant config/running Island/...}

• [ ] If applicable, add screenshots or log transcripts of the feature working

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 77.08%. Comparing base (13fb189) to head (8671e04).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## develop #4227 +/- ## ======================================== Coverage 77.08% 77.08% ======================================== Files 442 442 Lines 14137 14137 Branches 18 18 ======================================== Hits 10897 10897 Misses 3240 3240 ```

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