Explanation of void return in the functions documentation (#4590)

Co-authored-by: Laurenz <laurmaedje@gmail.com>

crates/typst/src/foundations/func.rs

@@ -81,6 +81,10 @@ pub use typst_macros::func;
 /// body evaluates to the result of joining all expressions preceding the
 /// `return`.
 ///
+/// Functions that don't return any meaningful value return [`none`] instead.
+/// The return type of such functions is not explicitly specified in the
+/// documentation. (An example of this is [`array.push`]).
+///
 /// ```example
 /// #let alert(body, fill: red) = {
 ///   set text(white)