docs: add spec of prefixes that are automaticly registered upon discovery of hub

Change-Id: I26a19e4c0f3608e0f159801e63e201d2d376d0ed
Refs: #2718
diff --git a/docs/manpages/ndn-autoconfig.rst b/docs/manpages/ndn-autoconfig.rst
index 8e56435..b436c61 100644
--- a/docs/manpages/ndn-autoconfig.rst
+++ b/docs/manpages/ndn-autoconfig.rst
@@ -38,6 +38,11 @@
 3.  Connect to the home NDN router according to user certificate.
     This ensures connectivity from anywhere.
 
+After connecting to an NDN router, two prefixes will be automatically registered:
+
+- /ndn
+- /localhop/nfd --- this to inform RIB manager that there is connectivity to the hub
+
 Stage 1: multicast discovery
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^