Add api ref link in TenantUsagesClient

This is to add api ref link in TenantUsagesClient.

Change-Id: I687fec348bd6ce9a0e3d3b3c62f7e6d743ceadb5
diff --git a/tempest/lib/services/compute/tenant_usages_client.py b/tempest/lib/services/compute/tenant_usages_client.py
index 5a748c7..d0eb1c9 100644
--- a/tempest/lib/services/compute/tenant_usages_client.py
+++ b/tempest/lib/services/compute/tenant_usages_client.py
@@ -24,6 +24,12 @@
 class TenantUsagesClient(base_compute_client.BaseComputeClient):
 
     def list_tenant_usages(self, **params):
+        """List Tenant Usage For All Tenants.
+
+        For a full list of available parameters, please refer to the official
+        API reference:
+        http://developer.openstack.org/api-ref/compute/#list-tenant-usage-for-all-tenants
+        """
         url = 'os-simple-tenant-usage'
         if params:
             url += '?%s' % urllib.urlencode(params)
@@ -34,6 +40,12 @@
         return rest_client.ResponseBody(resp, body)
 
     def show_tenant_usage(self, tenant_id, **params):
+        """Show Usage Details For Tenant.
+
+        For a full list of available parameters, please refer to the official
+        API reference:
+        http://developer.openstack.org/api-ref/compute/#show-usage-details-for-tenant
+        """
         url = 'os-simple-tenant-usage/%s' % tenant_id
         if params:
             url += '?%s' % urllib.urlencode(params)