Clarify group-id in tutorial.

doc/TUTORIAL.md

@@ -104,11 +104,12 @@ like this:
 
     [com.cedarsoft.utils.legacy/hibernate "1.3.4"]
 
-The group-id is the text before the slash and used to namespace artifacts.
-Especially for Java libraries, it's often a reversed domain name. Group ids are
-optional: some Clojure libraries (as with Lancet or Cheshire) omit group-id
-while others use them. If your project belongs to an organization, it is
-recommended to use reversed organization domain or organization name as group id.
+The group-id is the part before the slash. Especially for Java
+libraries, it's often a reversed domain name. Clojure libraries often
+use the same group-id and artifact-id (as with Lancet), in which case
+you can omit the group-id. If there is a library that's part of a
+larger group (such as `ring-jetty-adapter` being part of the `ring`
+project), the group-id is often the same across all the sub-projects.
 
 Sometimes versions will end in "-SNAPSHOT". This means that it is not
 an official release but a development build. Relying on snapshot