docs: clarify that AGS is throttled
docs: clarify that AGS is throttled

why:
* to answer questions from LTI partners about throttling
* since AGS requests are done with an institution-level token
instead of a user-level token, there is more of a danger
of getting throttled (though it hasn't happened yet)

closes INTEROP-7264
flag=none

test plan:
* no typos

Change-Id: I7a49ea06dcbaa54793931a221d830ba5866042e2
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/284676
Product-Review: Karl Lloyd <4c8060af93345c2cc416ee6aa55bceec4e5cc9d4@instructure.com>
Reviewed-by: Ryan Hawkins <867bc579926fa7e8ba296eacccde2d03d9fb556c@instructure.com>
Tested-by: Service Cloud Jenkins <9144042a601061f88f1e1d7a1753ea3e2972119d@instructure.com>
QA-Review: Xander Moffatt <27f70fe194da423604e6e0a63a3de19814736fa9@instructure.com>

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