MagicCombo: Document that the old API error message will be removed

I chose the date to be the same as the removal date of other compatibility stuff
recently introduced, so we'll only have one flag day. We could remove it
earlier, because the error was there for a while now, but I'd rather play it safe.

Fixes #374.

Signed-off-by: Gergely Nagy <algernon@keyboard.io>

doc/plugin/MagicCombo.md

@@ -83,6 +83,9 @@ and we'll help you with it.
  [gh:issues]: https://github.com/keyboardio/Kaleidoscope/issues
  [forums]: https://community.keyboard.io/
 
+Trying to use the old API errors out with a helpful error message, but as the
+old API is deprecated, this will be removed by 2019-01-14.
+
 ## The old API
 
 ```c++