Describe the handle-based custom queue APIs, worker-pool creation and
teardown, periodic requeue, cancellation semantics, and return values.
Clarify that libc user work queue APIs use blocking synchronization and
must only be called from task context, while kernel and Flat queue and
asynchronous cancellation operations remain ISR-safe.
Assisted-by: Codex:GPT-5
Signed-off-by: DuoYuWang <thirteenking.wang@gmail.com>