add description to graphql:subgraph:publish rake task
add description to graphql:subgraph:publish rake task

refs INTEROP-7004
flag = none

Change-Id: I9e091fc5a640d300ac26500f429aee0df4f6a3be
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/274568
Reviewed-by: Evan Battaglia <e557ffdfc4b80aab44858030b0ae5370b86cb7b1@instructure.com>
Tested-by: Service Cloud Jenkins <9144042a601061f88f1e1d7a1753ea3e2972119d@instructure.com>
QA-Review: Michael Ziwisky <ef1d6a5d3a192ea9486f58aa31b1688dee7f9f81@instructure.com>
Product-Review: Michael Ziwisky <ef1d6a5d3a192ea9486f58aa31b1688dee7f9f81@instructure.com>

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