Single-branch if shouldn't be replaced by when.

The readme says that if it finds the code `(if (some test) (some action) nil)` it will suggest replacing it with `while`. I think it means `when`, but this isn't right; single-branched `if` forms are totally acceptable if the focus is on the return value. Since `when` has an implicit `do` form, it is a way of signaling to readers that side-effects are involved, while `if` forms are all about what value is returned. Instead consider adding a rule for replacing `(if cond (do some action))` with `when`.

Start with the README section describing the single-branch if suggestion, then locate the existing rule that matches `(if (some test) (some action) nil)` and its surrounding behavior. Confirm whether the documentation or rule is wrong, and verify that the resulting suggestions distinguish return-value-oriented `if` forms from side-effect-oriented `(if cond (do some action))` forms.

Written by the indexing model from the issue text.