Add note to Vararg/UnionAll warning about making it an error (#56662)

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.

Author: mcabbott

src/jltypes.c

@@ -910,7 +910,7 @@ JL_DLLEXPORT jl_value_t *jl_type_unionall(jl_tvar_t *v, jl_value_t *body)
         if (jl_options.depwarn) {
             if (jl_options.depwarn == JL_OPTIONS_DEPWARN_ERROR)
                 jl_error("Wrapping `Vararg` directly in UnionAll is deprecated (wrap the tuple instead).\nYou 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)`.");
-            jl_printf(JL_STDERR, "WARNING: Wrapping `Vararg` directly in UnionAll is deprecated (wrap the tuple instead).\nYou 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)`.\n");
+            jl_printf(JL_STDERR, "WARNING: Wrapping `Vararg` directly in UnionAll is deprecated (wrap the tuple instead).\nYou 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)`.\nTo make this warning an error, and hence obtain a stack trace, use `julia --depwarn=error`.\n");
         }
         jl_vararg_t *vm = (jl_vararg_t*)body;
         int T_has_tv = vm->T && jl_has_typevar(vm->T, v);