Clarify that function definition needs to be in scope. Fixes #3003

[DJDuque]

Explicit mention that functions cannot be just defined "somewhere" e.g.

fn main() {                                                                      
    a_function();                                                                
                                                                                 
    {                                                                            
        fn a_function() {                                                        
            println!("Hello, World!");                                           
        }                                                                        
    }                                                                            
} 

See issue #3003