ObsoleteFiles.inc: Add comment regarding optional files

I had added entries that depended on some build option to this file
and have been informed, that those go into a different file in another
directory.

Mentioning /usr/src/tools/build/mk/OptionalObsoleteFiles.inc in this
file should help other committers (and me) to not repeat that mistake.

MFC after: 1 week

(cherry picked from commit 2a47875ea6a8359fd329ef1760da91b54396436f)