document range operation for array initialization

[eightfilms]

Found this neat trick on discord but it isn't in the docs, thought it might be useful - feel free to close otherwise

Yep. [?]int{0..<16 = 123} But note that if you want a slice, you'll probably need to do an explicit copy, or slice.fill

https://discord.com/channels/568138951836172421/568871298428698645/1269783875459088436