Document openstack complete help and usage

Currently we have no documentation for the `complete` command.

Change-Id: I9faf38341bb57a61869ddbef406a022219a7c62c
Closes-Bug: 1623860

Author: stevemar

doc/source/command-objects/complete.rst

@@ -0,0 +1,25 @@
+========
+complete
+========
+
+The ``complete`` command is inherited from the `python-cliff` library, it can
+be used to generate a bash-completion script. Currently, the command will
+generate a script for bash versions 3 or 4. The bash-completion script is
+printed directly to standard out.
+
+Typical usage for this command is::
+
+  openstack complete | sudo tee /etc/bash_completion.d/osc.bash_completion > /dev/null
+
+If installing ``python-openstackclient`` from a package (``apt-get`` or ``yum``),
+then this command will likely be run for you.
+
+complete
+--------
+
+print bash completion command
+
+.. program:: complete
+.. code:: bash
+
+    os complete