add documentation for xapi service

So that people know how to use it.

Test Plan:
 * It shows up in the api docs

Change-Id: I3f1c1d8418a9d722d2f43011c5839140922c0950
Reviewed-on: https://gerrit.instructure.com/43628
Reviewed-by: Braden Anderson <braden@instructure.com>
QA-Review: Braden Anderson <braden@instructure.com>
Product-Review: Bracken Mosbacker <bracken@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>