docs: add multitenat endpoints usage for vmalert multitenancy (#7665)

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docs/vmalert.md

@@ -474,6 +474,23 @@ There are the following approaches exist for alerting and recording rules across
   For example, `-remoteWrite.url=http://vminsert:8480/insert/123/prometheus` would write recording
   rules to `AccountID=123`.
 
+* To use the [multitenant endpoint](https://docs.victoriametrics.com/cluster-victoriametrics/#multitenancy-via-labels) of vminsert as
+  the `-remoteWrite.url` and vmselect as the `-datasource.url`, add `extra_label` with tenant ID as an HTTP URL parameter for each group.
+  For example, run vmalert using `-datasource.url=http://vmselect:8481/select/multitenant/prometheus -remoteWrite.url=http://vminsert:8480/insert/multitenant/prometheus`,
+  along with the rule group:
+```yaml
+groups:
+- name: rules_for_tenant_456:789
+  params:
+     extra_label: [vm_account_id=456,vm_project_id=789]
+  rules:
+    # Rules for accountID=456, projectID=789
+```
+
+The multitenant endpoint in vmselect is less efficient than [specifying tenants in URL](https://docs.victoriametrics.com/cluster-victoriametrics/#url-format).
+
+For security considerations, it is recommended restricting access to multitenant endpoints only to trusted sources, since untrusted source may break per-tenant data by writing unwanted samples or get access to data of arbitrary tenants.
+
 * To specify `tenant` parameter per each alerting and recording group if
   [enterprise version of vmalert](https://docs.victoriametrics.com/enterprise/) is used
   with `-clusterMode` command-line flag. For example: