update external tools API documentation for resource_selection
update external tools API documentation for resource_selection

clarify the interaction between it and not_selectable

closes #LA-100

Change-Id: I90f9e2cda08c8b443801ed093a6183f39cbb04a1
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/228340
Tested-by: Service Cloud Jenkins <9144042a601061f88f1e1d7a1753ea3e2972119d@instructure.com>
Reviewed-by: Clint Furse <b31c824c483447b2438702c2da6f729b3ebb9b50@instructure.com>
QA-Review: James Williams <3a5f4ed991dbfa5c9ef76ad729d240ca12f62180@instructure.com>
Product-Review: James Williams <3a5f4ed991dbfa5c9ef76ad729d240ca12f62180@instructure.com>

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