Help: Link from add_custom_target to policy on BYPRODUCTS

Link policy explaining BYPRODUCTS.

Apply the change from commit 00b788e85e (Help:custom_command: link
policy on BYPRODUCTS, 2022-09-09) to `add_custom_target` docs too.

Issue: #23786

Help/command/add_custom_target.rst

@@ -42,6 +42,8 @@ The options are:
   Each byproduct file will be marked with the :prop_sf:`GENERATED`
   source file property automatically.
 
+  *See policy* :policy:`CMP0058` *for the motivation behind this feature.*
+
   Explicit specification of byproducts is supported by the
   :generator:`Ninja` generator to tell the ``ninja`` build tool
   how to regenerate byproducts when they are missing.  It is