Add warning about unspecified join condition behaviour (#4276)

lib/ecto/query.ex

@@ -1279,6 +1279,16 @@ defmodule Ecto.Query do
   after the join. In the expression syntax, the options are given as
   the fifth argument.
 
+  > ### Unspecified join condition {: .warning}
+  >
+  > Leaving the `:on` option unspecified while performing a join
+  > that is not a cross join will trigger a warning. This is to
+  > help users avoid performing expensive cross joins when they don't
+  > mean to. If the behaviour is desired, you may remove the warning by
+  > changing to a cross join or explicitly setting `on: true`. If
+  > the behaviour is not desired, you should specify the appropriate
+  > join condition.
+
   ## Keywords examples
 
       from c in Comment,