Based on the various conversations that we've had (mostly documented in the etherpads links in T145918), we'd like to agree on the solutions to the questions that were raised by the team in order to do the cross-wiki search results.
1. Deployment/switchover and Cirrussearch configuration
** config problem is a problem anyway
*** we should fix it eventually (but we can probably delay it for quite a while)
2. Are there other ways to solve the problems that multi-project indices are supposed to solve?
** issue //synchronous// requests to one cluster
** issue //asynchronous// requests to one cluster
*** still might have CirrusSearch config problems for issuing a single query across indexes
** create small cluster (3-5 nodes) for small wikis
** We could run an A/B using the synchronous search version to see if people use it
3. Do people want autocomplete results from other wikis?
** how to do defaults
** prefixes are weird: https://en.wikipedia.org/wiki/s:San_Francisco
4. How do we enable cross-wiki search results per wiki?
** configuration switch to allow wiki's to 'turn off' the cross-wiki search if desired