Multiple clean-ups to the docs for builtin functions.

Multiple clean-ups to the docs for builtin functions.

* Use concrete example for dir() and eliminate the distracting doctest directives.
* Add a pure python equivalent for enumerate()
* Modify the enumerate() example to demonstrate the start argument
* Remove incorrect reference the *iterable* in the enumerate() docs.
* Downgrade the comments on input() from a warning to a note.
* Fix the iter() example to use the empty string as the terminating
  condition for file.readline().  Also, the old example was broken
  because readline() results include a newline, so 'STOP\n' would have
  been the correct terminating condition.  Even with that fix, the
  STOP example was fragile and would have lead to infinite loops with
  malformed inputs.
* Do not refer to classmethod as being "more advanced" than staticmethod.