MacBook Pro - How to resume APFS decryption process?
The process is totally automatic in High Sierra and there is no command to resume the decryption process other than connecting the Mac to power if it's a portable Mac.

The Apples new file system APFS works differently then older file systems HFS+. The detailed guideline along with new feature detail can be found here.

Now, whenever you'll unlock volume the pending/paused encryption/decryption process will start automatically. But there is a condition, the machine (in my case, MacBook Pro) must should be plugged in. So, whenever you'll unplug the decryption process will stop immediately even if the battery is fully charged since this is presumably a maintenance task that shouldn't deplete the battery energy at any time.