clarify user search API endpoint documentation
clarify user search API endpoint documentation

fixes CNVS-18647

call out that DB IDs will be matched first for numeric values, and that
even if it moves on to fuzzier search, it's not necessarily just (or
even including) SIS ID.

Change-Id: I968ce4eb20fb7c00afcfeee694c937b8b2d79ef1
test-plan: N/A
Reviewed-on: https://gerrit.instructure.com/53962
Tested-by: Jenkins
Reviewed-by: Ethan Vizitei <73eeb0b2f65d05a906adf3b21ee1f9f5a2aa3c1c@instructure.com>
QA-Review: August Thornton <9adc7a1161ddf32ff608de792a7e50179545f026@instructure.com>
Product-Review: Jacob Fugal <f862f167b85d41b225785c70d70808bc7337c1fe@instructure.com>

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