Added clarification of retention policy based on community feedback

No FAQ right now per discussion with Balaji. Addresses #231

content/v2.0/cloud/pricing-plans.md

@@ -28,6 +28,9 @@ Use this plan as much and as long as you want within the Free Plan rate limits:
 - **Writes:** 3MB every 5 minutes
 - **Query:** 30MB every 5 minutes
 - **Storage:** 72-hour data retention
+{{% note %}}
+Data retention is determined by the time at which data is written to InfluxDB; not the timestamp of the data point. You can write data with timestamps older than 72 hours, but 72 hours after that data is written, it is evicted.
+{{% /note %}}
 - **Series cardinality:** 10,000
 - **Create:**
   - Up to 5 dashboards