example conf: clarify group name parsing

conf/example.conf

@@ -24,9 +24,13 @@
 # syntax:
 #   @groupname = [one or more names]
 
-# groups let you club names together for convenience in specifying
-# permissions.  A group is simply expanded to whatever names are on the right
-# hand side when it is actually used
+# groups let you club (user or group) names together for convenience
+
+# * a group is like a #define in C except that it can *accumulate* values
+# * the config file is parsed in a single-pass, so later *additions* to a
+#   group name cannot affect earlier *uses* of it
+
+# The following examples should illustrate all this:
 
                 # you can have a group of people...
 @staff      =   sitaram some_dev another-dev
@@ -43,10 +47,14 @@
 @staff      =   au.thor
                 # so now "@staff" expands to all 4 names
 
-                # groups can include other groups (but not recursively)
+                # groups can include other groups, and the included group will
+                # be expanded to whatever value it currently has
 @interns    =   indy james
 @staff      =   bob @interns
                 # "@staff" expands to 7 names now
+@interns    =   han
+                # "@interns" now has 3 names in it, but note that this does
+                # not change @staff
 
 # REPO AND BRANCH PERMISSIONS
 # ---------------------------