Document public secret type
Document public secret type

Adds documentation for public secret types.  Add two examples of storing
public secrets, one with a POST followed by PUT, and one with a single
POST.  The second example is incomplete until bug 1441866 is fixed.

Change-Id: I5c892d1b09e6bc0cb155c5591b12f409d3c86419

openstack/barbican,openstack/barbican