doc: add release note on dead global map variable deletion

Add a short release note blurb on linker dead-coding of global map
variables.

Updates #58645.

Change-Id: Ibfdd3e2b2b78517db4245b1d7eedd5e75a643b47
Reviewed-on: https://go-review.googlesource.com/c/go/+/498575
Reviewed-by: Eli Bendersky <[email protected]>
TryBot-Bypass: Than McIntosh <[email protected]>

Author: thanm

doc/go1.21.html

@@ -190,9 +190,11 @@ <h2 id="linker">Linker</h2>
 </p>
 <!-- CL 463395 -->
 <p>
-  <!-- cmd/link: linker portion of dead map removal -->
+  In Go 1.21 the linker (with help from the compiler) is now capable of
+  deleting dead (unreferenced) global map variables, if the number of
+  entries in the variable initializer is sufficiently large, and if the
+  initializer expressions are side-effect free.
 </p>
-
 <p>
   TODO: complete this section, or delete if not needed
 </p>