Why does `diff()` method output positive/negative instead of absolute values?

**Describe the bug**
I always have a hard time fixing bugs, forgetting that the `diff()` method can be negative.
I think the problem is that the name of the method seems to get absolute values.

**Expected behavior**
What do you think about this?

**Information**
- Day.js Version [v1.11.9]

Start at the diff() API entry point and review its current behavior and documentation in the Day.js codebase. Determine whether the intended outcome is a behavior change, a renamed method, or clearer documentation; completion requires an agreed expectation and matching validation.

Written by the indexing model from the issue text.