[1.8.x] Added translation.override() context manager to docs.

Backport of cf34ee68 from master

docs/topics/i18n/translation.txt

@@ -1739,6 +1739,7 @@ will affect code running in the same thread.
 For example::
 
     from django.utils import translation
+
     def welcome_translated(language):
         cur_language = translation.get_language()
         try:
@@ -1757,6 +1758,16 @@ which returns the language used in the current thread,
 for the current thread, and ``django.utils.translation.check_for_language()``
 which checks if the given language is supported by Django.
 
+To help write more concise code, there is also a context manager
+``django.utils.translation.override()`` that stores the current language on
+enter and restores it on exit. With it, the above example becomes::
+
+    from django.utils import tranlations
+
+    def welcome_translated(language):
+        with translation.override(language):
+            return translation.ugettext('welcome')
+
 Language cookie
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