docs: add frequently asked questions to vmalert integration with vlogs

Signed-off-by: hagen1778 <roman@victoriametrics.com>

docs/VictoriaLogs/vmalert.md

@@ -205,4 +205,35 @@ requestDurationQuantile{stats_result="p00", service="service-2"}
 ...
 ```
 
-For additional tips on writing LogsQL, refer to this [doc](https://docs.victoriametrics.com/victorialogs/logsql/#performance-tips).
+For additional tips on writing LogsQL, refer to this [doc](https://docs.victoriametrics.com/victorialogs/logsql/#performance-tips).
+
+## Frequently Asked Questions
+
+* How to use [multitenancy](https://docs.victoriametrics.com/victorialogs/#multitenancy) in vmalert?
+  * vmalert doesn't support multi-tenancy for VictoriaLogs in the same way as it [supports it for VictoriaMetrics in ENT version](https://docs.victoriametrics.com/vmalert/#multitenancy).
+    However, it is possible to specify the queried tenant from VictoriaLogs datasource via `headers` param in [Group config](https://docs.victoriametrics.com/vmalert/#groups).
+    For example, the following config will execute all the rules within the group against tenant with `AccountID=1` and `ProjectID=2`:
+    ```yaml
+        groups:
+        - name: MyGroup
+          headers:
+          - "AccountID: 1"
+          - "ProjectID: 2"
+          rules: ...
+    ```
+* How to use one vmalert for VictoriaLogs and VictoriaMetrics rules in the same time?
+  * vmalert allows having many groups with different rule types (`vlogs`, `prometheus`, `graphite`).
+    But only one `-datasource.url` cmd-line flag can be specified, so it can't be configured with more than 1 datasource.
+    However, VictoriaMetrics and VictoriaLogs datasources have different query path prefixes, and it is possible to use [vmauth](https://docs.victoriametrics.com/vmauth/) to route requests of different types between datasources.
+    See example of vmauth config for such routing below:
+    ```yaml
+        unauthorized_user:
+          url_map:
+            - src_paths:
+              - "/api/v1/query.*"
+              url_prefix: "http://victoriametrics:8428"
+            - src_paths:
+              - "/select/logsql/.*"
+              url_prefix: "http://victorialogs:9428"
+    ```
+    Now, vmalert needs to be configured with `--datasource.url=http://vmauth:8427/` to send queries to vmauth, and vmauth will route them to the specified destinations as in configuration example above.