Doc: Add warning for not supported Prefix Delegation

Based on discussions during the Zed PTG (see [1]) mark Prefix Delegation
as experimental.
Add warning to cfg option ipv6_pd_enabled and to IPv6 documentation's
PD section that the feature has low test coverage, and dibbler is
concluded (the tool used for notifing Neutron that a prefix was changed)
see: [2].

[1]: https://etherpad.opendev.org/p/neutron-zed-ptg#L208
[2]: https://github.com/tomaszmrugalski/dibbler#project-status

Related-Bug: #1916428
Change-Id: Ia712eb6f5b6f947aeab89ba7c018771eb4104e37

doc/source/admin/config-ipv6.rst

@@ -488,6 +488,12 @@ endpoints can be accessed via an IPv6 network. At this time, Open vSwitch
 Prefix delegation
 ~~~~~~~~~~~~~~~~~
 
+.. warning::
+
+   This feature is experimental with low test coverage, and the Dibbler client
+   which is used for this feature is no longer maintained. For details see:
+   https://github.com/tomaszmrugalski/dibbler#project-status
+
 From the Liberty release onwards, OpenStack Networking supports IPv6 prefix
 delegation. This section describes the configuration and workflow steps
 necessary to use IPv6 prefix delegation to provide automatic allocation of

neutron/conf/common.py

@@ -66,7 +66,10 @@ core_opts = [
     cfg.IntOpt('max_subnet_host_routes', default=20,
                help=_("Maximum number of host routes per subnet")),
     cfg.BoolOpt('ipv6_pd_enabled', default=False,
-                help=_("Enables IPv6 Prefix Delegation for automatic subnet "
+                help=_("Warning: This feature is experimental with low test "
+                       "coverage, and the Dibbler client which is used for "
+                       "this feature is no longer maintained! "
+                       "Enables IPv6 Prefix Delegation for automatic subnet "
                        "CIDR allocation. "
                        "Set to True to enable IPv6 Prefix Delegation for "
                        "subnet allocation in a PD-capable environment. Users "