The first time? It would be similar to a high five type situation. Like “hey, did you see that? Hell yeah!!!”
A warning for guys? I wouldn’t look at it like that. Be proud of it – fucking brag about it. So many girls want to squirt but can’t so I wouldn’t be ashamed or feel you need to warn a guy. Inform them? Sure.
Either way, squirting is hot but not required so just let him know it may happen, then relax and enjoy yourself.