"Vectorized function": demystify at first use

[jennybc]

This term can sound much more intimidating than it is, especially if you are new to R. @edgararuiz likened it to "quantum entanglement".

How to demystify it quickly when introduced?

@garrett suggestions: "a function that works with vectors", a diagram, clear code demo

@jimhester points to this quote from @andrie's book:

Vectorized functions are a very useful feature of R, but programmers who are used to other languages often have trouble with this concept at first. A vectorized function works not just on a single value, but on a whole vector of values at the same time.