[docs] multiprocessing documentation of Process.exitcode

[Josh Kupershmidt]

Hi all,

The documentation for the multiprocessing module claims:

| Note that the start(), join(), is_alive() and exit_code methods
| should only be called by the process that created the process object.

Two problems with this description:
 1. "exit_code" should be spelled "exitcode"
 2. exitcode is an attribute of the Process class, not a method as
claimed, since the underlying method is decorated with @property

Also, aside from the above documentation issues, if it is true that
it's not safe to query the exitcode from a separate process, should
the exitcode property method include a check like so:

    assert self._parent_pid == os.getpid(), 'can only test a child process'

since start(), join(), and is_alive() all perform this check?

Josh