The next-gen auto_attribs api documentation does not clearly describe
cases where (a) only a subset of attributes are annotated and (b)
`field` definitions are provided for a subset of fields. Update
docstring to reflect current, desirable, behavior.

Add tests to clarify `.define` behavior focused on fully-annotated
classes with partially-defined fields, which is commonly used to add
non-default behavior to a subset of a classes fields. For example:

```python

@attr.define
class NewSchool:
    x: int
    y: list = attr.field()

    @y.validator
    def _validate_y(self, attribute, value):
        if value < 0:
            raise ValueError("y must be positive")

```

The previous docstring *could* be read to imply that:

* The new-school API will not infer auto_attribs if there are any
  unannotated attributes.
* The new-school API will not infer auto_attribs if *any* attr.ib are
  defined, even if those attr.ibs are type annotated.