Help:custom_command: link policy on BYPRODUCTS

Link policy explaining BYPRODUCTS.

Fixes: #23786

Help/command/add_custom_command.rst

@@ -67,6 +67,8 @@ 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

Help/policy/CMP0058.rst

@@ -71,6 +71,9 @@ every custom command dependency, even on source files, needs to
 be treated this way because CMake does not have enough information
 to know which files are generated as byproducts of custom commands.
 
+Introducing Byproducts
+^^^^^^^^^^^^^^^^^^^^^^
+
 CMake 3.2 introduced the ``BYPRODUCTS`` option to the
 :command:`add_custom_command` and :command:`add_custom_target`
 commands.  This option allows byproducts to be specified explicitly: