Add documentation for new GraphQL rake tasks

The following discussion from !52318 (merged) should be addressed:

• @reprazent started a discussion: (+1 comment)

  @alexkalderimis Handy! I think we should document the existence of these rake tasks though, I'm not sure if they belong next to the other GraphQL rake tasks or in the style guide.

  Happy to do that in a separate MR though.

  I also had one non-blocking thought on how we pass arguments in !52318 (diffs, comment 502658624)