per: https://bugs.python.org/issue38401

There is not a way to document fields of a dataclass.

I propose that instead of making a language change, that an additional parameter to the field be added in similar vein to property.

This currently works:
```
class Foo:
 def getx(self):
  return 5
 x = property(getx, doc='document the x property')
```

So, I propose this:
```
@dataclass
class Bar:
 x : int = field(doc='document what x is')
```

This should be easy to support as I believe that the above would not require any changes to the core language.