[user-guide]Update the content of log-in to the dashboard

According to this commit 0e2337ec21,
the mutil-domain credentials is set by default for Liberty. But the
End User Guide covers various release. Therefore, we need enter the
domain, user name and password, if the cloud enables the multi-domain
credentials.

Change-Id: Icb441717fc49b039c79eef608b6efa89de6e5c1b
Closes-Bug: #1554906

doc/common/log_in_dashboard.rst

@@ -6,7 +6,8 @@ The dashboard is generally installed on the controller node.
 
 #. Ask the cloud operator for the host name or public IP address from
    which you can access the dashboard, and for your user name and
-   password.
+   password. If the cloud supports multi-domain model, you also need to
+   ask for your domain name.
 
 #. Open a web browser that has JavaScript and cookies enabled.
 
@@ -30,7 +31,8 @@ The dashboard is generally installed on the controller node.
       exception in the browser to bypass the warning.
 
 #. On the Log In page, enter your user name and password, and click
-   :guilabel:`Sign In`.
+   :guilabel:`Sign In`. If the cloud supports multi-domain model, you also
+   need to enter your domain name.
 
    The top of the window displays your user name. You can also access the
    :guilabel:`Settings` tab (:ref:`dashboard-settings-tab`) or sign out
@@ -60,7 +62,8 @@ Within a project, a user creates and manages instances.
 
 From the :guilabel:`Project` tab, you can view and manage the resources in a
 selected project, including instances and images. You can select the project
-from the drop down menu at the top left.
+from the drop down menu at the top left. If the cloud supports multi-domain
+model, you can also select the domain from this menu.
 
 .. figure:: figures/dashboard_project_tab.png
    :width: 100%