add note in api documentation about 64 bit ids
add note in api documentation about 64 bit ids

closes CNVS-7220

test plan
- should have "All ids in canvas are 64 bit integers"

Change-Id: I45ed55f85233c48aee38194695fc04c6e73ab623
Reviewed-on: https://gerrit.instructure.com/22803
Tested-by: Jenkins <d95b56ce41a2e1ac4cecdd398defd7414407cc08@instructure.com>
Reviewed-by: Cody Cutrer <c4fe2b1d90ef8f2548c8aeabfa633434316f0305@instructure.com>
Product-Review: Cody Cutrer <c4fe2b1d90ef8f2548c8aeabfa633434316f0305@instructure.com>
QA-Review: Cody Cutrer <c4fe2b1d90ef8f2548c8aeabfa633434316f0305@instructure.com>

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