* guix/scripts/system.scm (display-system-generation, list-generations):
  New procedures.
  (process-action): Clarify docstring.
  (process-command): New procedure.
  (guix-system)[parse-sub-command]: Add 'list-generations'
  Call 'process-command' instead of 'process-action'.
* doc/guix.texi (Using the Configuration System): Mention generations,
  rollback, and 'list-generations'.
  (Invoking guix system): Document 'list-generations'.