In this edit, I added a spoiler and that was the main function of the comment (and therefore would have been the comment has it not been a spoiler). Should I reveal what the spoiler is in the comment? I was unsure, so I used "added spoiler, I'm not revealing what it said in the edit comment" as the comment. Is this correct in how I should handle spoiling edit descriptions?
I think your approach is fine.
Edit descriptions don't matter all that much.
If you're the OP editing your own post, or if you have over 2k rep and can edit anyone's post, most likely nobody will even look at the edit description; you can (and many people do) just leave the default text of "added 74 characters in body" or whatever.
The only time people are likely to read your edit description is if the edit goes to the Suggested Edits queue for approval (if you're a <2k rep user editing someone else's post). Even then, the most important thing is the actual changes you made to the post, which will be clearly visible to reviewers. I imagine even many reviewers don't bother to check the edit description.
The only time I usually bother to include edit descriptions is when I'm making an edit whose purpose might not be immediately clear: to explain to the OP, or to others who might see the revision history, why I'm making that edit and why it improves the post. If you're just editing your own post to clarify a point or add a caveat, I wouldn't say you even need an edit description at all.
All this being said, if you want to put an edit description, and you're concerned about spoilers, then better safe than sorry: go ahead and write something like "added a spoilery caveat" as you did. It doesn't hurt to have a less descriptive edit description, because people can look and see what you changed anyway. (It probably won't hurt to have a spoilery one either, since most people won't even bother to see the revision history, but you might as well avert that small possibility if you can.)