Added the limitations of return type function

guide/english/c/functions/index.md

@@ -123,4 +123,5 @@ If the speed of the program is important then you may not want to use recursion
 * Functions take parameters to work with- if they're taking nothing, use `void`.
 * `return` ends the function and gives back a value. You can have several in one function, but as soon as you hit one the function ends there.
 * When you pass a variable to a function, it has its own copy to use - changing something in a function doesn't change it outside the function.
+* A function can return any type of values, except array or any other function.
 * Variables declared inside a function are only visible inside that function, unless they are declared static.