add API doc for 'select' param in content_exports
add API doc for 'select' param in content_exports

the context_export API can export a list of files and folders, not
just the default "everything" for a course, group, or user. This
was not documented, though it was being used.

refs ADMIN-972

test plan:
- Check Content Export API documentation for POST method
- Ensure the use of select makes sense and looks usable

Change-Id: I12d8e4a19fb0aeca56ea318ef9d71da887ccaf98
Reviewed-on: https://gerrit.instructure.com/148041
Tested-by: Jenkins
Reviewed-by: Mysti Sadler <431d062aff1e8f25416b3a7b732fe6effdb03f9a@instructure.com>
Reviewed-by: Jeremy Stanley <b3f594e10a9edcf5413cf1190121d45078c62290@instructure.com>
QA-Review: Mysti Sadler <431d062aff1e8f25416b3a7b732fe6effdb03f9a@instructure.com>
Product-Review: Carl Kibler <7c2b40129dd885a147d5b2dda56dda016b9a8f57@instructure.com>

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