config: update sharding.yaml

This patch updates example for vshard usage in config module.

NO_DOC=example update
NO_TEST=example update
NO_CHANGELOG=example update

doc/examples/config/sharding.yaml

 # See the vshard documentation for proper usage of vshard. The basic scenario
 # with this configuration is described below.
 #
-# Before using sharding, you need to create a spaces that will contain data and
-# define functions to work with these spaces on the storage masters, for
-# example:
+# After the replicasets have been booted, but before using sharding, you need to
+# create spaces that will contain the data and define functions to work with
+# these spaces on the master storages. For example:
 #
 # format = {{'id', 'unsigned'}, {'bucket_id', 'unsigned'}}
 # s = box.schema.space.create('test', {format = format})
@@ -42,19 +42,20 @@ credentials:
     replicator:
       password: '{{ context.replicator_password }}'
       roles: [replication]
-     # For now, vshard itself will grant the specified user all the permissions
-     # it needs to prepare instances for sharding. However, they will be revoked
-     # after reload(). Since there is no way to grant the user  the proper
-     # rights to the functions, the "super" role is used.
     storage:
       password: '{{ context.storage_password }}'
-      roles: [super]
+      # The default 'sharding' role is required by storage users. It has all the
+      # necessary privileges for sharding.
+      roles: [sharding]
 
 iproto:
   listen:
   - uri: 'unix/:./{{ instance_name }}.iproto'
-  # We currently need to manually define the user for vshard. If the user is not
-  # defined, the guest is used.
+  # If a login is not specified, 'guest' is used. If a login is specified and a
+  # user with that name is described in credentials.users, then the user should
+  # have the default 'sharding' role. If a login is specified and the user is
+  # not described in credentials.users, the user's existence and privileges will
+  # be checked when the vshard storage is started.
   advertise:
     peer:
       login: 'replicator'