Return types, pass by reference vs pass by value

[iyves]

Expanded on what data types can be returned. Also added section for the differences between passing an object as a parameter by value versus by reference.

• [ x ] I have read freeCodeCamp's contribution guidelines.
• [ x ] My pull request has a descriptive title (not a vague title like Update index.md)
• [ x ] My pull request targets the master branch of freeCodeCamp.
• [ x ] None of my changes are plagiarized from another source without proper attribution.
• [ x ] My article does not contain shortened URLs or affiliate links.

If your pull request closes a GitHub issue, replace the XXXXX below with the issue number.

Closes #XXXXX

[moT01]

looks good @iyves, thanks for contributing!