Document that @spawnat :any doesn't do load-balancing

Instead we point users to `WorkerPool`, which will do load-balancing.

Author: JamesWrigley

src/macros.jl

@@ -56,8 +56,11 @@ end
 
 Create a closure around an expression and run the closure
 asynchronously on process `p`. Return a [`Future`](@ref) to the result.
+
 If `p` is the quoted literal symbol `:any`, then the system will pick a
-processor to use automatically.
+processor to use automatically. Using `:any` will not apply any form of
+load-balancing, consider using a [`WorkerPool`](@ref) and [`remotecall(f,
+::WorkerPool)`](@ref) if you need load-balancing.
 
 # Examples
 ```julia-repl