I was in iMessage when I accidentally typed "eag" which is a shortcut I have created to easily insert my work email in iOS. Well I typed that and pressed space and the text autofilled into the message. So then I went ahead and typed other shortcuts I actively use in iOS and sure enough, they all auto-complete very nicely. After noticing this, I went to System Preferences » Keyboard » Text and found an entire Menu that you can control this (and I assume will also sync to iCloud Keychain).
[crarko adds: I'm pretty sure I've seen this mentioned before, but in case it is a new and useful fact to somebody here it is.]

