MFC Note: ${FILES} documentation change omitted since r299094 will

		not be MFCed to ^/stable/10 .

MFC r320442:

share/examples/tests/{atf,plain}/Makefile: tweak example Makefile snippets

• Including bsd.own.mk isn't required since no MK_<foo> knobs are being manipulated.
• Update documentation to note that ${FILES} is installed via bsd.progs.mk, not bsd.prog.mk.