docs: custom status message

docs/advanced/offline/CustomOnlineStatusMessage.md

@@ -0,0 +1,46 @@
+# Custom Connection Status Message
+
+By default, the header bar shows either an "Online" or "Offline" message based on whether the app can connect to the DHIS2 server. If it's useful for an app to share other relevant info in that badge, for example "3 changes saved locally", then it's possible to set a custom message there:
+
+![Custom online status message](custom-online-status-message.png)
+
+## Hooks
+
+```ts
+import {
+    useSetOnlineStatusMessage,
+    useOnlineStatusMessageValue,
+    useOnlineStatusMessage,
+} from '@dhis2/app-runtime'
+```
+
+### `useSetOnlineStatusMessage`
+
+It's most likely that you'll only use `useSetOnlineStatusMessage()`, a hook that returns a `setState` function. It can set the value in the Header Bar without causing a rerender for itself.
+
+`setOnlineStatusMessage()` accepts a `ReactNode`-type object, so icons or other components can be added to the badge, but don't go crazy. A simple string is also a valid `ReactNode`:
+
+```ts
+const setOnlineStatusMessage: (message: ReactNode) => void =
+    useSetOnlineStatusMessage()
+```
+
+### `useOnlineStatusMessageValue`
+
+This hook returns just the value:
+
+```ts
+const onlineStatusMessage: ReactNode = useOnlineStatusMessageValue()
+```
+
+### `useOnlineStatusMessage`
+
+This hook returns both the value and the `set` function:
+
+```ts
+const { onlineMessage, setOnlineStatusMessage } = useOnlineStatusMessage()
+```
+
+## Example
+
+You can see an example in use in the [Aggregate Data Entry app](https://github.com/dhis2/aggregate-data-entry-app/blob/dadd61392ea010a8017a19a25eaf76f885d9eea7/src/data-workspace/use-handle-headerbar-status.js)

docs/advanced/offline/useDhis2ConnectionStatus.md

@@ -6,6 +6,8 @@ This hook is used to detect whether or not the app can connect to the DHIS2 serv
 
 It's designed to detect server connection because checking just the internet connection can lead to problems on DHIS2 instances that are implemented offline, where features or actions might be blocked because the device is offline, even though the app can connect to the DHIS2 server just fine. In these cases, what matters is whether or not the app can connect to the DHIS2 server.
 
+This what the DHIS2 Header Bar uses to show an "Online" or "Offline" badge.
+
 ```ts
 import { useDhis2ConnectionStatus } from '@dhis2/app-runtime'