Fix shell completion issues and documentation

This issue was raised in #990. The completer was never used in Python3.4+,
because it was overwritten by Python's completer. Oddly enough it has always
worked in Python2.7/3.3.

The documentation was also slightly modified. os.path.join was always a
complex beast.

jedi/api/replstartup.py

@@ -11,8 +11,8 @@ Then you will be able to use Jedi completer in your Python interpreter::
     [GCC 4.6.1] on linux2
     Type "help", "copyright", "credits" or "license" for more information.
     >>> import os
-    >>> os.path.join().split().in<TAB>                     # doctest: +SKIP
-    os.path.join().split().index   os.path.join().split().insert
+    >>> os.path.join('a', 'b').split().in<TAB>            # doctest: +SKIP
+    ...dex   os.path.join().split().insert
 
 """
 import jedi.utils