added clarity to schema:download

[mwarger]

Hopefully less pain moving from apollo-codegen.

[apollo-cla]

@mwarger: Thank you for submitting a pull request! Before we can merge it, you'll need to sign the Meteor Contributor Agreement here: https://contribute.meteor.com/

[jeffwillette]

I think you also have to add it to the CLI code here

https://github.com/apollographql/apollo-cli/blob/7d99a31be25b42d24284b90b30c0ae7ade365d2a/packages/apollo-cli/src/commands/schema/download.ts#L31

[jbaxleyiii]

@mwarger thanks so much for this helpful addition!