Custom metadata for top-level '(a <- foo(x))' being used to induce printing

Have seen a few hits on `implicit_assignment_linter()` like:

```r
(a <- foo(x))
```

Here, the lint message can mention explicitly using `print()` for readability rather than this shorthand.

We should of course be careful not to mix this case which is similar on the parse tree:

```
AA && (BB <- foo(x))
```