Update cohorts.mdx (#1883)

* Update cohorts.mdx

Call out added for Cohorts Section - Historical Event Import Queries (Please hold on auto-merging for now; We should merge once the modifications are live)

* Update cohorts.mdx

changing the section for the call out to be under "How Cohorts are Computed"

Author: Tofufu

pages/docs/users/cohorts.mdx

@@ -103,6 +103,10 @@ When breaking down metrics by multiple cohorts, events are evaluated independent
 
 ## How Cohorts are Computed
 
+<Callout type="info">
+    **Note:** If you import historical data with events timestamped more 30 days ago, the historical events will not be used in your existing cohort queries until 12 hours after you've imported the data.
+</Callout>
+
 Cohorts are computed dynamically at the time that you use them in a query. Suppose you make a funnels report that filters down to the New Users cohort. Under the hood, Mixpanel does the following:
 1. Run a query to compute the set of users who are in the cohort as of right now (the time that the query is run).
 2. Run a query to compute the set of users who converted in the funnel, filtering down to the list produced in Step 1.