api-ref: document the 'tenant_id' query parameter

When listing servers, it's possible to filter servers based on
the tenant_id. This patch documents this.

Change-Id: I8432aa922cb61ef052cd1f7436546e69d7b6aceb

api-ref/source/parameters.yaml

@@ -717,6 +717,16 @@ tags_query:
     all tags in this list will be returned. Boolean expression in this
     case is 't1 AND t2'. Tags in query must be separated by comma.
   min_version: 2.26
+tenant_id_query:
+  description: |
+    When passed alongside the ``all_tenants=1`` query parameter, the
+    ``tenant_id`` query parameter only returns the resources that belong to the
+    specified tenant. By default this is only allowed by admin users. Note that
+    this query parameter has no effect and is ignored unless the
+    ``all_tenants=1`` query parameter is also passed.
+  in: query
+  required: false
+  type: string
 usage_limit:
   description: |
     Requests a page size of items. Calculate usage for the limited number of

api-ref/source/servers.inc

@@ -167,6 +167,7 @@ Request
   - ip: ip_query
   - reservation_id: reservation_id_query
   - all_tenants: all_tenants_query
+  - tenant_id: tenant_id_query
   - deleted: deleted_query
   - ip6: ip6_query
   - tags: tags_query
@@ -471,6 +472,7 @@ Request
   - ip: ip_query
   - reservation_id: reservation_id_query
   - all_tenants: all_tenants_query
+  - tenant_id: tenant_id_query
   - deleted: deleted_query
   - ip6: ip6_query
   - tags: tags_query