Add search time thresholds to handbook
As per the new test for elasticsearch record insertion and recall that suggests bug prioritization on failing (!39130 (merged)), we should update the handbook to indicate what those thresholds are so that future test writers can incorporate those thresholds into future tests by looking at the handbook instead of looking at test files.
Thresholds should be added here https://about.gitlab.com/handbook/engineering/quality/issue-triage/#severity, of if not there, then we should link to a new page in https://about.gitlab.com/handbook/engineering/development/enablement/search that does have thresholds.
Edited by Erick Banks