It does it via your schema definition, like if you want to document an argument on a field (to use a specific example):
field do
arg :size, :integer, description: "The size of the thing"
arg :name, :string, description: "The desired name of the thing"
end
Or document an overall ‘type’:
@desc """
The [`DateTime`](https://hexdocs.pm/elixir/DateTime.html) scalar type represents a date and time in the UTC
timezone. The DateTime appears in a JSON response as an ISO8601 formatted
string, including UTC timezone ("Z"). The parsed date and time string will
be converted to UTC and any UTC offset other than 0 will be rejected.
"""
scalar :datetime, name: "DateTime" do
serialize &DateTime.to_iso8601/1
parse &parse_datetime/1
end
Etc… It’s all the docs (though perhaps a dedicated section for just this purpose, if it doesn’t already exist, would be useful). 
