clarify limitation for /series endpoint

Signed-off-by: Jakob Kartschall <[email protected]>

Author: Jakob Kartschall

docs/configuration/v1-guarantees.md

@@ -24,6 +24,7 @@ The Cortex maintainers commit to ensuring future version of Cortex can read data
 Cortex strives to be 100% API compatible with Prometheus (under `/api/prom/*`); any deviation from this is considered a bug, except:
 
 - Requiring the `__name__` label on queries.
+- For queries to the `/api/v1/series`, `/api/v1/labels` and `/api/v1/label/{name}/values` endpoints, given time ranges older than the data stored in ingesters will be ignored and data will only retrieved from ingesters.
 - Additional API endpoints for creating, removing and modifying alerts and recording rules.
 - Additional API around pushing metrics (under `/api/push`).
 - Additional API endpoints for management of Cortex itself, such as the ring.  These APIs are not part of the any compatibility guarantees.