add IDx User Id docs

[rtoscani]

Added IDx documentation for User ID. prebid/Prebid.js#5826

[jdwieland8282]

@rtoscani please add an entry to download.md

[rtoscani]

@jdwieland8282 done

[jdwieland8282]

Hi @rtoscani I'm not getting a cookie (or LS value) when I build your branch. I do see a sharedid cookie though.

         "userSync":{
            "userIds":[{
                  name: 'sharedId',
                  params: {
                            syncTime: 60 // in seconds, default is 24 hours
                   },
                  storage: {
                      name: 'sharedid',
                      type: 'html5',
                      expires: 28
                  },
              },
              {
                name:'idx'
              }]
         }
      });

[rtoscani]

Hi @jdwieland8282, we do not generate the IDx from prebid right now (only reading existing IDx is implemented in prebid), the only way of generating a cookie is using our tag (and it's on beta right now). We are making a lot of changes in our API, in the next PR you will be able to generate and use an IDx from prebid.

[jdwieland8282]

I see thanks for clarifying. You may want to add that to your documentation, but it's up to you.

[jdwieland8282]

noted that the module only reads cookies that exist and does not create news ones. I've suggested this distinction be added to the documentation but am not requiring it.

[bretg]

part of 4.12