docs: kselftest: Document the FORCE_TARGETS build variable

docs: kselftest: Document the FORCE_TARGETS build variable

FORCE_TARGETS has been part of the kselftest build system for
some time but is absent from the developer documentation. Without
an entry here, users relying on kselftest in CI pipelines would
have to read the selftests Makefile directly to discover the
option.

A build that exits zero despite some targets failing can mask
real breakage and mislead automated systems into reporting
success. Add a dedicated section so that CI authors can easily
find and adopt FORCE_TARGETS=1 to turn such silent partial
failures into hard errors.

Link: https://lore.kernel.org/r/20260417-selftests-docs-v1-1-32e4a78214eb@suse.com
Signed-off-by: Ricardo B. Marlière <rbm@suse.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>