[core] Add a note for the Y.config.core documentation

[andrewnicols]

No description provided.

[caridy]

I'm not so sure that msg reflects the reality about core. In fact, I don't recommend people replacing that thru YUI_config directly, because we might change what is in that list and it will break their app. Instead, we recommend to do this:

YUI.Env.core.push.apply(YUI.Env.core, YUI_config.extendedCore || []);
YUI(); // first instance in the page...

that does the trick, adding new stuff into core from a new config variable called extendedCore (which is not YUI specific), and does not break or mess with the original core definition.