add descriptions of black net news private groups

docs/net-news.md

@@ -23,6 +23,11 @@ Once you are connected you can subscribe to any of the available
 groups that interest you. When you launch the program again at
 a later date, new postings in those groups will show up.
 
+The program can also download descriptions of the groups by using
+the `-d` flag.
+
+    $ slrn -d
+
 ## Groups
 
 All groups beginning with `tilde.*` are part of the public Tilde
@@ -36,8 +41,34 @@ system.
 
 ### black.general
 
+The general channel is used for announcements and updates about
+Tilde Black. Users that have questions for the admin or need help
+with something on the server can make their requests here.
+
 ### black.privacy
 
+The privacy channel is intended for discussions around
+experiments, ideation, and documentation that promotes privacy
+online.
+
 ### black.anonymity
 
+The anonymity channel is a playground for protecting identities
+online. Theories, troubleshooting, sharing of tools and guides, or
+the creation of new documents are all great subjects to include
+here.
+
 ### black.security
+
+Discussions on security can get highly technical and detailed. We
+strive to keep those in this group where the chains can go as deep
+as they need to reach agreement. Ultimately the goal of the
+security group is to make suggestions for the public on best
+practices but suggestions for the improvement of security on Tilde
+Black is also welcome.
+
+This is not a black or grey-hat hacking group and any discussions
+along those lines will result in account termination. Tilde Black
+is a collaborative space designed to welcome discussions and
+experimentation, but the server itself is not designed as an
+exploit target or port-scanning test box.