Update the docs to show that Clearance does not handle flash messages with Users#create

I had to do some digging before I realized that Clearance doesn't handle flash messages for creation like it does for other interactions. I would have liked the documentation to reflect this, so I added it.

Hopefully, this will be useful for others using Clearance.

Issue that led me to make the change: https://github.com/thoughtbot/clearance/issues/204?source=cc