Document how to use loopback devices for Swift

People commonly want to use loopback devices to evaulate or develop on
Swift.  The steps for this are slightly different than those for
physical block devices.

Also make some small fix ups to the title formatting.

Change-Id: I0c1cb4a261f264dc56bd065ab137e5ee79e773f8

doc/swift-guide.rst

@@ -11,8 +11,8 @@ Before running Swift we need to generate "rings", which are binary compressed
 files that at a high level let the various Swift services know where data is in
 the cluster. We hope to automate this process in a future release.
 
-disks with partition table (recommended)
-~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+Disks with a partition table (recommended)
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
 Swift also expects block devices to be available for storage. To prepare a disk
 for use as Swift storage device, a special partition name and filesystem label
@@ -30,8 +30,22 @@ Follow the example below to add 3 disks for an AIO demo setup.
         (( index++ ))
     done
 
-disks without partition table
-~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+For evaluation, loopback devices can be used in lieu of real disks:
+
+::
+
+    index=0
+    for d in sdc sdd sde; do
+        free_device=$(losetup -f)
+        fallocate -l 1G /tmp/$d
+        losetup $free_device /tmp/$d
+        parted $free_device -s -- mklabel gpt mkpart KOLLA_SWIFT_DATA 1 -1
+        sudo mkfs.xfs -f -L d${index} ${free_device}p1
+        (( index++ ))
+    done
+
+Disks without a partition table
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
 Kolla also supports unpartitioned disk (filesystem on /dev/sdc instead of
 /dev/sdc1) detection purely based on filesystem label. This is generally not a
@@ -46,7 +60,7 @@ ansible/roles/swift/defaults/main.yml
     swift_devices_match_mode: "prefix"
     swift_devices_name: "swd"
 
-rings
+Rings
 ~~~~~
 
 Run following commands locally to generate Rings for AIO demo setup. The