text: Title: Add doc strings to argument annotation classes
  Body: Argument Annotation classes in `uplink/types.py` are missing class doc strings. To improve code documentation, we need to add doc strings to the following classes, adhering the [Google Style Guide](https://google.github.io/styleguide/pyguide.html?showone=Comments#Comments) for consistency with the rest of the codebase:
  
  - [x] `uplink.types.Query`
  - [x] `uplink.types.QueryMap`
  - [x] `uplink.types.Header`
  - [x] `uplink.types.HeaderMap`
  - [x] `uplink.types.Field`
  - [x] `uplink.types.FieldMap`
  - [x] `uplink.types.Part`
  - [x] `uplink.types.PartMap`
  - [x] `uplink.types.Body`
  - [x] `uplink.types.Url`