Update docs index page

- Switch from proposing Ubuntu 16.04 to 18.04 as the most tested
  platform.
- Make it clearer that creating an additional "stack" user is
  optional when running on a cloud image, as this step often leads
  to errors for new users.
- Fix some minor nits along the way.

Change-Id: I39aef1a230b668b932b1681fcd0deeb423b411f1
diff --git a/doc/source/index.rst b/doc/source/index.rst
index 9186f6d..1ea1c5d 100644
--- a/doc/source/index.rst
+++ b/doc/source/index.rst
@@ -38,22 +38,23 @@
 Install Linux
 -------------
 
-Start with a clean and minimal install of a Linux system. Devstack
+Start with a clean and minimal install of a Linux system. DevStack
 attempts to support the two latest LTS releases of Ubuntu, the
 latest/current Fedora version, CentOS/RHEL 7, as well as Debian and
 OpenSUSE.
 
-If you do not have a preference, Ubuntu 16.04 is the most tested, and
-will probably go the smoothest.
+If you do not have a preference, Ubuntu 18.04 (Bionic Beaver) is the
+most tested, and will probably go the smoothest.
 
-Add Stack User
---------------
+Add Stack User (optional)
+-------------------------
 
-Devstack should be run as a non-root user with sudo enabled
+DevStack should be run as a non-root user with sudo enabled
 (standard logins to cloud images such as "ubuntu" or "cloud-user"
 are usually fine).
 
-You can quickly create a separate `stack` user to run DevStack with
+If you are not using a cloud image, you can create a separate `stack` user
+to run DevStack with
 
 .. code-block:: console
 
@@ -76,12 +77,12 @@
    $ cd devstack
 
 The ``devstack`` repo contains a script that installs OpenStack and
-templates for configuration files
+templates for configuration files.
 
 Create a local.conf
 -------------------
 
-Create a ``local.conf`` file with 4 passwords preset at the root of the
+Create a ``local.conf`` file with four passwords preset at the root of the
 devstack git repo.
 
 .. code-block:: ini