Deleting shared memory segment with shmctl
Your first reasoning is correct. The shared segment will exists until both: it is marked with IPC_RMID and last process detaches.
The purpose of the second fragment is to remind you, that in a solution using shared memory you need to have some process mark it for destruction or it will remain in memory/swap forever. It might be good idea to use IPC_RMID immediately after creating segment.
If you are not sure you have successfully released memory, you can use ipcs program to list remaining segments.