Remove unused arg 'defer' in 'job_start' #1000

* With the changes in commit
  "events: Refactor how event deferral is handled"
  (2e4ea29d2c) the function argument
  'defer' of 'job_start' and member variable 'defer' of 'struct job'
  can be removed.
* Update/Fix the documentation for function 'job_start'.

src/nvim/eval.c

@@ -10498,7 +10498,6 @@ static void f_job_start(typval_T *argvars, typval_T *rettv)
             on_job_stdout,
             on_job_stderr,
             on_job_exit,
-            true,
             0,
             &rettv->vval.v_number);