Help: Add notes for topic 'custom-command-byproducts'

Help/release/dev/custom-command-byproducts.rst

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+custom-command-byproducts
+-------------------------
+
+* The :command:`add_custom_command` and :command:`add_custom_target`
+  commands learned a new ``BYPRODUCTS`` option to specify files
+  produced as side effects of the custom commands.  These are not
+  outputs because they do not always have to be newer than inputs.