Documentation: KUnit: Fix non-uml anchor

The section was rewritten but its anchor got left behind.
Fix the anchor and add some references to running on QEMU.

Signed-off-by: Tales Aparecida <tales.aparecida@gmail.com>
Reviewed-by: David Gow <davidgow@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>