text: Title: Overriding `formats`, `export_formats`, `import_formats` not documented. 
  Body: I wanted to restrict valid formats differently for different models.
  
  By examining the code I discovered that I could set `formats`, `export_formats`, or `import_formats` as attrs on the admin to control the allowed formats. 
  
  This is not documented. 
  
  I am not sure if this was an intended feature, but it is useful enough that I recommend it be promoted to such, and documented. 
  