docs: adds note about dynamic config (#4882)

* docs: adds note about dynamic config

* less technical verbiage

* removes `dynamic configuration` verbiage

* list all replaceable values

docs/docs/configuration/index.md

@@ -36,6 +36,23 @@ It is not recommended to copy this full configuration file. Only specify values
 
 :::
 
+**Note:** The following values will be replaced at runtime by using environment variables
+
+- `{FRIGATE_MQTT_USER}`
+- `{FRIGATE_MQTT_PASSWORD}`
+- `{FRIGATE_RTSP_USER}`
+- `{FRIGATE_RTSP_PASSWORD}`
+
+for example:
+```yaml
+mqtt:
+  user: "{FRIGATE_MQTT_USER}"
+  password: "{FRIGATE_MQTT_PASSWORD}"
+```
+```yaml
+- path: rtsp://{FRIGATE_RTSP_USER}:{FRIGATE_RTSP_PASSWORD}@10.0.10.10:8554/unicast
+```
+
 ```yaml
 mqtt:
   # Optional: Enable mqtt server (default: shown below)