add comment outlining test.

Author: pnkfelix

src/test/run-pass/borrowck/issue-62007-assign-field.rs

@@ -1,4 +1,9 @@
 // run-pass
+
+// Issue #62007: assigning over a field projection (`list.0 = n;` in
+// this case) should be able to kill all borrows of `list.0`, so that
+// `list.0` can be borrowed on the next iteration through the loop.
+
 #![allow(dead_code)]
 
 struct List<T> {