Add note to Vararg/UnionAll warning about making it an error

[mcabbott]

This warning message

WARNING: Wrapping `Vararg` directly in UnionAll is deprecated (wrap the tuple instead).
You may need to write `f(x::Vararg{T})` rather than `f(x::Vararg{<:T})` or `f(x::Vararg{T}) where T` instead of `f(x::Vararg{T} where T)`.

(last extended in #49558) seems clear enough if you wrote the code. But if it's coming from 10 packages deep, there's no information to track down the origin.

Turns out you can do this with --depwarn=error. But the message doesn't tell you that, and doesn't call itself a deprecation warning at all.

[nsajko]

I wonder if it'd be possible to add this message to all deprecation warning automatically, instead of hardcoding it here?

[oscardssmith]

I believe this does happen for most deprecation messages, but this one comes from C so needs to do things itself.

[mcabbott]

Right. Until I looked at the code, I had no idea this warning shared anything at all with Base.depwarn -- it makes no claim to be a deprecation.

I don't think the warnings from Base.depwarn tell you how to make them into errors, by default. Maybe they should but not this PR:

julia> Base.depwarn("msg", :fun; force=true)
┌ Warning: msg
│   caller = ip:0x0
└ @ Core :-1