No, the reader must not need to read the function to understand how it works.

The idea would be that the function names are so good that one does not need to read the function contents, right?

But some things are complicated and may not be expressible with the function name alone.

That's why people write comments. To help readers understand the code.

Let's say I am implementing some algorithm. I may mention the performance characteristics in the comment. Or I may let the readers dig into the code to find out what they are. Seriously, please don't follow "no comments" rule dogmatically.