doc: document new panic behavior of flag definition after Set

For #57411

Change-Id: I56c112bb03dde24c2e2643c9b72ce06158a8e717
Reviewed-on: https://go-review.googlesource.com/c/go/+/499278
TryBot-Bypass: Keith Randall <khr@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Keith Randall <khr@google.com>

doc/go1.21.html

@@ -456,7 +456,9 @@ <h3 id="minor_library_changes">Minor changes to the library</h3>
     </p>
 
     <p><!-- CL 480215 -->
-      TODO: <a href="https://go.dev/cl/480215">https://go.dev/cl/480215</a>: flag: panic if a flag is defined after being set
+      A flag definition (via <code>Bool</code>, <code>BoolVar</code>, <code>Int</code>, <code>IntVar</code>, etc.) will panic if <code>Set</code> has already been called on a flag with the same name.
+      <p>
+      This change is intended to detect cases where <a href="#language">changes in initialization order</a> cause flag operations to occur in a different order than expected. In many cases the fix to this problem is to introduce a explicit package dependence to correctly order the definition before any <code>Set</code> operations.
     </p>
   </dd>
 </dl><!-- flag -->