Clarify function definitions vs. expressions

Fixes #2980.

Author: chriskrycho

src/ch03-03-how-functions-work.md

@@ -125,7 +125,8 @@ assigning a value to it with the `let` keyword is a statement. In Listing 3-1,
 </Listing>
 
 Function definitions are also statements; the entire preceding example is a
-statement in itself.
+statement in itself. (As we will see below, *calling* a function is not a
+statement.)
 
 Statements do not return values. Therefore, you can’t assign a `let` statement
 to another variable, as the following code tries to do; you’ll get an error: