Document removal of xacml-pdp API limitation

In the Honolulu Maintenance release the decision API is always running

In Istanbul the: REST server is always running (i.e., the port is always
responsive), but the decision API is enabled/disabled by PDP-STATE-CHANGE
messages from PAP.

Jim Hahn asked me to document this.

Issue-ID: POLICY-3556
Change-Id: I6eee0b2f3d1e03b09dda7e14e3fa528826dc0d2b
Signed-off-by: liamfallon <liam.fallon@est.tech>
diff --git a/docs/xacml/decision-api.rst b/docs/xacml/decision-api.rst
index 7d068c1..8f6a993 100644
--- a/docs/xacml/decision-api.rst
+++ b/docs/xacml/decision-api.rst
@@ -9,6 +9,10 @@
 specific situation. The Decision API mimics closely the XACML request standard in that it supports a subject, action
 and resource.
 
+When the PAP activates an xacml-pdp, the decision API becomes available. Conversely, when the PAP deactivates an xacml-pdp, the
+decision API is disabled. The decision API is enabled/disabled by the PDP-STATE-CHANGE messages from PAP. If a request is made
+to the decision API while it is deactivated, a "404 - Not Found" error will be returned.
+
 .. csv-table::
    :header: "Field", "Required", "XACML equivalent", "Description"