man: Clarify usage of /usr/share/factory/ in programs

man: Clarify usage of /usr/share/factory/ in programs

As discussed in this thread:
https://github.com/redhat-performance/tuned/issues/798#issuecomment-3197697654

/usr/share/factory/ is not intended to be read from by programs,
but the wording in the FHS can be misread to think that programs
should be using /usr/share/factory/ as the vendor supplied configuration
directory rather than something like /usr/lib/foo/ or /usr/share/foo/.

This commit points developers to the UAPI configuration spec for how to
make their programs hermetic /usr/ compatible.

Signed-off-by: Ryan Brue <ryanbrue.dev@gmail.com>