Update Prebid Video dev docs to note that cache is toggle-able

[rmloveland]

You can show video ads even if cache is disabled. However, there are some conditions:

• In general, video-enabled bidders must supply either bid.vastUrl or bid.vastXml on their responses, and they may supply both.
• If you have Prebid Cache disabled, and the bidder supplies only bid.vastXml in its bid response, dfp.buildVideoUrl will not be able to generate a video ad tag URL from that response, and it will be dropped from the auction.
• If options.url is passed as an argument to dfp.buildVideoUrl:
  • If Prebid Cache is enabled, Prebid does not set the description_url field to the bid response's bid.vastUrl. It just attaches the bid's ad server targeting and builds the URL based on user input.
  • If Prebid Cache is disabled, Prebid sets the description_url field to the bid response's bid.vastUrl.

In addition, there is some clarification needed about the status of Prebid Cache (open/closed source, who hosts it, etc.). Right now it's being treated as somewhat magical so we need to address.

[rmloveland]

Related: #422

[jbartek25]

Hi @rmloveland,

You can show video ads even if cache is disabled.

Would you mind explaining how? There's no example of Prebid instream without cache I could find on prebid.org and even the ad ops documentation suggests to set up DFP creatives with redirects to the cache.

If Prebid Cache is disabled, Prebid sets the description_url field to the bid response's bid.vastUrl.

How does this allow me to play the ad?

The only way I can think of how to set up instream without the cache is to create a separate set of line items for each bidder with redirects to bidders' VAST url and ad ID macro. I would be grateful if you could suggest a better way. Thanks.