update the documentation regarding the `get_fixture` helper

please note that the japanese translation was done using "google translate" and should probably be checked again... :)

--HG--
branch : doctest-fixtures

doc/en/doctest.txt

@@ -50,3 +50,9 @@ then you can just invoke ``py.test`` without command line options::
     mymodule.py .
     
     ========================= 1 passed in 0.02 seconds =========================
+
+It is possible to use fixtures using the ``get_fixture`` helper::
+
+    # content of example.rst
+    >>> tmp = get_fixture('tmpdir')
+    >>> ...

doc/ja/doctest.txt

@@ -72,3 +72,12 @@ Python モジュール (通常 python テストモジュールを含む) の doc
     mymodule.py .
     
     ========================= 1 passed in 0.02 seconds =========================
+
+..
+    It is possible to use fixtures using the ``get_fixture`` helper::
+
+それは ``get_fixture`` ヘルパーを使ってフィクスチャを使用することが可能である::
+
+    # content of example.rst
+    >>> tmp = get_fixture('tmpdir')
+    >>> ...