Yes, I am not building a language. But some of my library's functions simply behave that way to allow for arbitrary amounts of data to be provided and at any rate, not limited by the amount of data that can be physically present on one invocation of the function (think about a network socket, for instance, you don't know how much data you are going to receive next, you could receive it all in one shot, or one byte each time you check, so you need a way to make it work the same regardless of how much you've got every time you poll the socket - there are other examples of course). Actually everything is quite consistent overall.
How does the documentation for std::cin describe it?
It doesn't really say anything, actually. The std::cout version says it "inserts" into the stream. I think the notion of stream might help me describe it, though.