Documentation: Mention Vararg{Any, N} in the "Performance Tips" sec…

…tion of the manual (#34796)

doc/src/manual/performance-tips.md

@@ -534,6 +534,11 @@ but this will:
 g_vararg(x::Vararg{Int, N}) where {N} = tuple(x...)
 ```
 
+One only needs to introduce a single type parameter to force specialization, even if the other types are unconstrained. For example, this will also specialize, and is useful when the arguments are not all of the same type:
+```julia
+h_vararg(x::Vararg{Any, N}) where {N} = tuple(x...)
+```
+
 Note that [`@code_typed`](@ref) and friends will always show you specialized code, even if Julia
 would not normally specialize that method call. You need to check the
 [method internals](@ref ast-lowered-method) if you want to see whether specializations are generated