Adds comprehensive documentation for the POSIX three-tier user identity
model (real, effective, saved-set IDs) enabled by CONFIG_SCHED_USER_IDENTITY.
* Updates sched/Kconfig with detailed help text explaining the config.
* Adds user_identity.rst to formally document credential inheritance
and the privilege transition rules for setuid(), seteuid(), setgid(),
and setegid().
* Updates tasks_vs_threads.rst to list credentials as a shared task
group resource.
Signed-off-by: Abhishek Mishra <mishra.abhishek2808@gmail.com>
