Clarify "owned data" in E0515.md

This clarifies the explanation of why this is not allowed and also what to do instead.

Fixes 62071

PS There was suggestion of adding a link to the book. I did not yet do that, but if desired that could be added.

Author: hkBst

compiler/rustc_error_codes/src/error_codes/E0515.md

@@ -18,9 +18,11 @@ fn get_dangling_iterator<'a>() -> Iter<'a, i32> {
 ```
 
 Local variables, function parameters and temporaries are all dropped before the
-end of the function body. So a reference to them cannot be returned.
+end of the function body. A returned reference (or struct containing a reference)
+to such a dropped value would immediately become invalid. Therefore it is not
+allowed to return such a reference.
 
-Consider returning an owned value instead:
+Consider returning a value that takes ownership of local data instead:
 
 ```
 use std::vec::IntoIter;