Should interface names begin with an “I” prefix?

Question

I have been reading "Clean Code" by Robert Martin to hopefully, become a better programmer. While none of it so far has been really ground breaking it has made me think differently about the way I design applications and write code.

There is one part of the book that I not only don't agree with, but doesn't make sense to me, specifically in regards to interface naming conventions. Here's the text, taken directly from the book. I have bolded the aspect of this I find confusing and would like clarification on.

I prefer to leave interfaces unadorned. The preceding I, so common in today’s legacy wads, is a distraction at best and too much information at worst. I don’t want my users knowing that I’m handing them an interface.

Perhaps it is because I'm only a student, or maybe because I have never done any professional or team based programming but I would want the user to know it is an interface. There's a big difference between implementing an interface and extending a class.

So, my question boils down to, "Why should we hide the fact that some part of the code is expecting an interface?"

Edit

In response to an answer:

If your type is an interface or a class is your business, not the business of someone using your code. So you shouldn't leak details of your code in this thrid party code.

Why should I not "leak" the details of whether a given type is an interface or a class to third-party code? Isn't it important to the third-party developer using my code to know whether they will be implementing an interface or extending a class? Are the differences simply not as important as I'm making them out to be in my mind?