If the lines of code were already very readable, they wouldn't require a comment. Comments are for interfaces or complex or inobvious code that can't easily be made more obvious simply through an expression of code IMO.

The code example of the OP is excellent because there are literally so many more avenues for code readability that are all superior to comments. First of all, the same code can literally be reused in the next line:

    addScrollbar(HORIZONTAL);
    addScrollbar(VERTICAL);

Do you frequently or always need to add both scrollbars to the UI? Another chance for reducing repetition and error-proneness when refactoring:

    addScrollbars();

There's no increase in mental load because chances are you'll never need to navigate into these functions to know what they are doing at all, you can read over them as you would normally when you read the "This adds a horizontal scrollbar" comment, and trust that the implementation does what it says on the tin.