Update ad unit reference with multiformat info

[rmloveland]

• get adunit reference PR merged
• make sure to capture validations from multiformat size validation checks
• add multi-format example after multiformat docs PR is in (UPDATE: already there in https://github.com/prebid/prebid.github.io/pull/558/files#diff-628e29f35528d1cbf2536e328e5afb51R201)