Edit define-class doc

Edit define-class doc

* doc/ref/goops.texi (Defining New Classes): Remove "fixme" text
  (saying we should have something that we in fact already have).  A
  few minor edits throughout.

  (Basic Class Definition): Move content up into `Defining New
  Classes'.

  (Class Options): Move after `Slot Options' (because slot options are
  more important).  Remove doc of the #:environment option, since I
  believe it's now fictitious.

  (Slot Options): Reorder the options so that the most commonly used
  ones come first.