Link to documentation in the cross-db Exception

This change updates the exception message for the cross-database
modification error to include a link to the documentation.

spec/support/database/prevent_cross_database_modification.rb

@@ -101,7 +101,8 @@ module Database
       if schemas.many?
         raise Database::PreventCrossDatabaseModification::CrossDatabaseModificationAcrossUnsupportedTablesError,
           "Cross-database data modification of '#{schemas.to_a.join(", ")}' were detected within " \
-          "a transaction modifying the '#{all_tables.to_a.join(", ")}' tables"
+          "a transaction modifying the '#{all_tables.to_a.join(", ")}' tables." \
+          "Please refer to https://docs.gitlab.com/ee/development/database/multiple_databases.html#removing-cross-database-transactions for details on how to resolve this exception."
       end
     end
   end