Improve transparency of 5_i32 versus 5i32

Author: wparad

src/scope/borrow.md

@@ -21,6 +21,8 @@ fn borrow_i32(borrowed_i32: &i32) {
 
 fn main() {
     // Create a boxed i32, and a stacked i32
+    // Remember: numbers can have arbitrary underscores added for readibility
+    // 5_i32 is the same as 5i32
     let boxed_i32 = Box::new(5_i32);
     let stacked_i32 = 6_i32;