Updating docs to get ROSETTA_LANGUAGE_GROUPS=True working about django-rosetta HOT 1 OPEN

OK, I was testing something else, and realised that if my user is BOTH in the translators-sr and the generic translators group, suddenly my user sees only the languages he has been assigned to.

Therefore, I guess the the help should be modified from:

ROSETTA_LANGUAGE_GROUPS: Set to True to enable language-specific groups, which can be used to give different translators access to different languages. Instead of creating a global translators group, create individual per-language groups, e.g. translators-de, translators-fr, and assign users to these.

to
ROSETTA_LANGUAGE_GROUPS: Set to True to enable language-specific groups, which can be used to give different translators access to different languages. In addition of creating a global translatorsgroup, create individual per-language groups, e.g.translators-de, translators-fr, and assign users to **both** the global translators group and the language-specific groups they need access to.

from django-rosetta.