Reflect that 'field_value_factor' is only in 1.2.x

While the blogpost http://www.elasticsearch.org/blog/2014-04-02-this-week-in-elasticsearch/ states, that feature #5519 was
added to 1.x, the release notes for, e.g. v1.1.2, however tell otherwise.
Only the release notes for 1.2.0 list #5519 as a new feature.

Since the 1.x docs deprecate/discourage from using `_boost`, and seemingly give a migration example at
http://www.elasticsearch.org/guide/en/elasticsearch/reference/current/mapping-boost-field.html#function-score-instead-of-boost
users of 1.1.x should be warned.