docs: Add workaround instructions in upgrade guide for API changes

Follow up to #13421

Signed-off-by: Christian Haudum <christian.haudum@gmail.com>

docs/sources/setup/upgrade/_index.md

@@ -36,6 +36,14 @@ The output is incredibly verbose as it shows the entire internal config struct u
 
 ## Main / Unreleased
 
+### HTTP API
+
+The API endpoint for instant queries `/api/v1/query` now returns a HTTP status 400 (Bad Request) when the provided `query`
+parameter contains a log selector query instead of returning inconsistent results. Please use the range query endpoint
+`/api/v1/query_range` (`Range` type in Grafana Explore) instead.
+
+### Configuration
+
 Loki changes the default value of `-ruler.alertmanager-use-v2` from `false` to `true`. Alertmanager APIv1 was deprecated in Alertmanager 0.16.0 and is removed as of 0.27.0.
 
 ## 3.0.0