text: Title: Add doc strings to method annotation classes
  Body: Method annotation classes in `uplink/decorators.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.decorators.headers`
  - [x] `uplink.decorators.form_url_encode`
  - [x] `uplink.decorators.multipart`
  - [x] `uplink.decorators.json`
  - [x] `uplink.decorators.timeout`
  - [x] `uplink.decorators.args`