docs: add a blurb in the single-vm doc about cloud-init log output

I had to poke around for awhile to find /var/log/cloud-init-output.log
so I figured I'd add a sentence in the docs about using that.

Change-Id: I8bb6cb730032e41661ee443da816cbea2b28f76d

Author: Matt Riedemann

doc/source/guides/single-vm.rst

@@ -78,6 +78,11 @@ passed as the user-data file when booting the VM.
 As DevStack will refuse to run as root, this configures ``cloud-init``
 to create a non-root user and run the ``start.sh`` script as that user.
 
+If you are using cloud-init and you have not
+`enabled custom logging <../configuration.html#enable-logging>`_ of the stack
+output, then the stack output can be found in
+``/var/log/cloud-init-output.log`` by default.
+
 Launching By Hand
 -----------------