Warn that replying to an escalated flag replies to the flagger, not the escalating mod

[luap42]

I just handled my first flag escalation and added a short reply to the action. From the form, I'd have presumed, that the reply goes to the moderator who escalated the flag, but instead the reply goes to the user who flagged initially. While this will hopefully only cause some mild confusions about why I mentioned an "escalation" in my reply, there should be a warning shown that you are replying to the flagger, not the mod, because otherwise staff might accidentally leak sensitive information intended for the mod.

Suggested text:

Attention: The reply to the flag will be shown to the flagger, not the mod escalating this flag. Do not share sensitive, mod-only information.

[cellio]

I'm assuming this isn't too hard? I didn't want to presume; @luap42 please change the complexity label as appropriate.