Clarify Vararg/UnionAll deprecation warning (#49558)

Fixes #49553

Author: timholy

src/jltypes.c

@@ -831,8 +831,8 @@ JL_DLLEXPORT jl_value_t *jl_type_unionall(jl_tvar_t *v, jl_value_t *body)
     if (jl_is_vararg(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).");
-            jl_printf(JL_STDERR, "WARNING: Wrapping `Vararg` directly in UnionAll is deprecated (wrap the tuple instead).\n");
+                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_vararg_t *vm = (jl_vararg_t*)body;
         int T_has_tv = vm->T && jl_has_typevar(vm->T, v);