fix test case for _() in templates, improve documentation

[Antti Kaihola]

[source:django/trunk/tests/regressiontests/templates/tests.py#6996#L739 the Test case for using the _() construct in templates] is invalid and doesn't actually test the feature at all:

'i18n10': ('{{ bool|yesno:_("ja,nein") }}', {'bool': True}, 'ja'),

This should be better, and it does pass:

'i18n10': ('{{ bool|yesno:_("yes,no,maybe") }}', {'bool': True, 'LANGUAGE_CODE': 'de'}, 'Ja'),

Also, ​the documentation for using _() in templates could clarify that |yesno:_("yes,no") used in the example actually looks for the string "yes,no" in the translation files, not for "yes" and "no" separately.

It would be better to use an actual existing translation string like "yes,no,maybe" instead of "yes,no" in the example.

[Antti Kaihola]

fix for the test case (same as in bug description)

[Antti Kaihola]

suggested improvement for documentation

[Malcolm Tredinnick]

I'm not going to apply the documentation patch, since it's very fragile for somebody's code to rely on what is a "standard" Django string. That will change from time to time. They will pick up things they mark for translation in their own app translation file.

However, a clarification about the whole string being looked up should go in. I'll fix that.

[Malcolm Tredinnick]

(In [7059]) Clarified that filter arguments are passed as a single string to the translation support. Refs #6326.

[Malcolm Tredinnick]

(In [7060]) Fixed #6326 -- Made the test for translations of filter arguments test something that has a chance of failure. Thanks akaihola.