text: Title: export-schema does not include ENUM descriptions
  Body: <!--- Provide a general summary of the changes you want in the title above. -->
  allow `strawberry  export-schema` to use, say, first line of docstring (or full docstring) as a comment and store it as part of the `schema.graphql` file  - for entities and for attributes. 
  
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  ## Feature Request Type
  
  - [ ] Core functionality
  - [ ] Alteration (enhancement/optimization) of existing feature(s)
  - [x] New behavior
  
  ## Description
  
  Our Federated GQL schema requires each attribute to have comments.
  It seems that currently there is no way to auto-add docstrings to the exported schema file via strawberry.