Start documenting the fields of the raw type YAML files and what they
mean, to make it easier to write new raw types.
Change-Id: I9672b757e89f1cc665d7e90078d83cfd87173d7f
Reviewed-on: https://cl.tvl.fyi/c/depot/+/3865
Reviewed-by: grfn <grfn@gws.fyi>
Tested-by: BuildkiteCI
