Clarify runtime representation of Unit (#211)

* Clarify the representation of `Unit`

* Fix typo

* Clarify explanation more

* Clarify explanation more, yet again

Author: triallax

text/chapter10.md

@@ -1136,6 +1136,8 @@ For simple types, the correspondence is almost trivial. For example, if an expre
 
 A similar law holds for expressions of type `Int` `Number` and `String` - expressions of type `Int` or `Number` evaluate to non-null JavaScript numbers, and expressions of type `String` evaluate to non-null JavaScript strings. Expressions of type `Int` will evaluate to integers at runtime, even though they cannot not be distinguished from values of type `Number` by using `typeof`.
 
+What about `Unit`? Well, since `Unit` has only one inhabitant (`unit`) and its value is not observable, it doesn't actually matter what it's represented with at runtime. Old code tends to represent it using `{}`. Newer code, however, tends to use `undefined`. So, although it doesn't really matter what you use to represent `Unit`, it is recommended to use `undefined` (not returning anything from a function also returns `undefined`).
+
 What about some more complex types?
 
 As we have already seen, PureScript functions correspond to JavaScript functions of a single argument. More precisely, if an expression `f` has type `a -> b` for some types `a` and `b`, and an expression `x` evaluates to a value with the correct runtime representation for type `a`, then `f` evaluates to a JavaScript function, which when applied to the result of evaluating `x`, has the correct runtime representation for type `b`. As a simple example, an expression of type `String -> String` evaluates to a function which takes non-null JavaScript strings to non-null JavaScript strings.