functions.md: state that the default return type is the unit type

Author: mattheww

src/items/functions.md

@@ -42,10 +42,11 @@
 > [^fn-param-2015]: Function parameters with only a type are only allowed
 >   in an associated function of a [trait item] in the 2015 edition.
 
-A _function_ consists of a [block], along with a name and a set of parameters.
+A _function_ consists of a [block], along with a name, a set of parameters, and an output type.
 Other than a name, all these are optional.
 Functions are declared with the keyword `fn`.
 Functions may declare a set of *input* [*variables*][variables] as parameters, through which the caller passes arguments into the function, and the *output* [*type*][type] of the value the function will return to its caller on completion.
+If the output type is not explicitly stated, it is the [unit type].
 
 When referred to, a _function_ yields a first-class *value* of the corresponding zero-sized [*function item type*], which when called evaluates to a direct call to the function.
 
@@ -387,6 +388,7 @@ fn foo_oof(#[some_inert_attribute] arg: u8) {
 [block]: ../expressions/block-expr.md
 [variables]: ../variables.md
 [type]: ../types.md#type-expressions
+[unit type]: ../types/tuple.md
 [*function item type*]: ../types/function-item.md
 [Trait]: traits.md
 [attributes]: ../attributes.md