Fix documentation for joins across clusters

We weren't able to get our gem for splitting across clusters into Rails
for 6.1. We're hoping to have that work completed for 7.0 but I should
have never promised in the docs to get it done by a specific time.

Ref #41537

guides/source/active_record_multiple_databases.md

@@ -447,9 +447,9 @@ handles outside of Rails.
 
 ### Joining Across Databases
 
-Applications cannot join across databases. Rails 6.1 will support using `has_many`
+Applications cannot join across databases. At the moment applications will need to
-relationships and creating 2 queries instead of joining, but Rails 6.0 will require
+manually write two selects and split the joins themselves. In a future version Rails
-you to split the joins into 2 selects manually.
+will split the joins for you.
 
 ### Schema Cache