text: Title: Add Field Description and example to Predefined Documents
  Body: ### Initial Checks
  
  - [X] I have searched Google & GitHub for similar requests and couldn't find anything
  - [X] I have read and followed [the docs](https://docs.docarray.org) and still think this feature is missing
  
  ### Description
  
  One nice thing about DocArray is the predefined Document types that we offer that allow people to use for multimodal applications.
  
  Having rich field definitions, with description and examples would help when building services based on FastAPI with them.
  
  ### Affected Components
  
  - [ ] [Vector Database / Index](https://docs.docarray.org/user_guide/storing/docindex/)
  - [X] [Representing](https://docs.docarray.org/user_guide/representing/first_step)
  - [X] [Sending](https://docs.docarray.org/user_guide/sending/first_step/)
  - [ ] [storing](https://docs.docarray.org/user_guide/storing/first_step/)
  - [X] [multi modal data type](https://docs.docarray.org/data_types/first_steps/)