SF bug #501591: dir() doc is old

Bugfix candidate.

+ Updated dir() description to match actual 2.2 behavior.

+ Replaced the dir(sys) example with dir(struct), because the former
  was way out of date and is bound to change frequently, while the
  latter is stable.

+ Added a note cautioning that dir() is supplied primarily for
  convenience at an interactive prompt (hoping to discourage its
  use as the foundation of introspective code outside the core).