Explain why a function with less arguments can be assigned to a variable declared as a function type that has more arguments

[adriancuadrado]

Page URL: https://github.com/microsoft/TypeScript/wiki/FAQ#why-are-functions-with-fewer-parameters-assignable-to-functions-that-take-more-parameters

Issue: We already have a section called "Assignability of Functions" in the handbook. I think it would be appropriate to move the wiki section there so it gets more visibility.