add backup option to the copy modules docs

rst/modules/copy.rst

@@ -15,9 +15,16 @@ module.
 +--------------------+----------+---------+----------------------------------------------------------------------------+
 | dest               | yes      |         | Remote absolute path where the file should end up                          |
 +--------------------+----------+---------+----------------------------------------------------------------------------+
+| backup             | no       | no      | Create a backup file including the timestamp information so you can        |
+|                    |          |         | get the original file back if you somehow clobbered it incorrectly.        |
++--------------------+----------+---------+----------------------------------------------------------------------------+
 | OTHERS             |          |         | All arguments the file module takes are also supported                     |
 +--------------------+----------+---------+----------------------------------------------------------------------------+
 
 Example action from Ansible :doc:`playbooks`::
 
     copy src=/srv/myfiles/foo.conf dest=/etc/foo.conf owner=foo group=foo mode=0644
+
+Copy a new ntp.conf file into place, backing up the original if it differs from the copied version::
+
+    copy src=/srv/myfiles/ntp.conf dest=/etc/ntp.conf owner=root group=root mode=644 backup=yes