kernel: work: document error returns for schedule operations
kernel: work: document error returns for schedule operations

When a non-zero delay is used the schedule functions can't fail, but
if K_NO_WAIT is passed error conditions may be forwarded from
k_submit_to_queue.

Also add a missed error return from k_submit_to_queue.

Signed-off-by: Peter Bigot <a4112a160b558d5427af084318038e19a95979ee@nordicsemi.no>

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