firewalldb: clarify bbolt kvstores illustration

During the migration of the kvstores to SQL, we'll iterate over the
buckets in the bbolt database, which holds all kvstores records.

In order to understand why the migration iterates over the buckets in
the specific order, we need to clarify the bbolt kvstores illustration
docs, so that it correctly reflects how the records are actually stored
in the bbolt database.

Author: ViktorT-11

firewalldb/kvstores_kvdb.go

@@ -16,13 +16,13 @@ the temporary store changes instead of just keeping an in-memory store is that
 we can then guarantee atomicity if changes are made to both the permanent and
 temporary stores.
 
-rules -> perm -> rule-name -> global   -> {k:v}
-              -> sessions -> group ID  -> session-kv-store  -> {k:v}
-			               -> feature-kv-stores -> feature-name -> {k:v}
+"rules" -> "perm" -> <rule-name> -> "global" -> {k:v}
+                                 -> "session-kv-store" -> <group-ID> -> {k:v}
+                                                                     -> "feature-kv-stores" -> <feature-name> -> {k:v}
 
-      -> temp -> rule-name -> global   -> {k:v}
-	      -> sessions -> group ID  -> session-kv-store  -> {k:v}
-				       -> feature-kv-stores -> feature-name -> {k:v}
+        -> "temp" -> <rule-name> -> "global" -> {k:v}
+                                 -> "session-kv-store" -> <group-ID> -> {k:v}
+                                                                     -> "feature-kv-stores" -> <feature-name> -> {k:v}
 */
 
 var (