Issue Board documentation needs to be updated

Issue Board documentation (https://docs.gitlab.com/ee/user/project/issue_board.html) is deprecated, as described here: http://disq.us/p/1rgjmxs.

Multiple Issue Board is now available in Starter and Premium tiers (not only in Ultimate).

There are also some differences in how Multiple Issue Boards can be used depending on what tier you are using: https://docs.gitlab.com/ee/user/project/issue_board.html#features-per-tier.