clarify(?) :key documentation
From:

    For +many_to_one+ associations, is the foreign_key in current
    model's table that references associated model's primary key, as a
    symbol.

    To:

        For +many_to_one+ associations, this is the foreign_key in the
        current model's table that references the associated model's
        primary key as a symbol.

Merge branch 'devel'