bug workaround when saving an event handler

Due to https://github.com/fish-shell/fish-shell/issues/845 the functions handling events are not activated upon load. Funcsave of the event handlers doesn't lead to proper results and should be avoided.
I'd prefer something like this:

> You can define your function in `.config/fish/config.fish` or put it anywhere Fish will see it before it needs to run. (Note: saving it with `funcsave` won't work.)

Any more detail than that is making the docs longer than they need to be, IMO.