Differentiate between currying and partial application

[schaloner]

2 issues:

• The current documentation doesn't give an example of partial application
• The example given for currying is that of a Function2, which gives the same result as partial application for that specific case.

This PR adds a section for partial application and updates the section on currying, giving higher arity examples in both cases.

[danieldietrich]

Hi Steve, thank you for improving the documentation and making the difference between currying and partial application clear. Well done!