Add precision about stateful iterators to isempty

The manual now mentions that stateful iterators should implement `isdone`,
so it appears `isempty` is not supposed to consume elements.
This matters a lot for callers as if you can't rely on this you can't do
`isempty(c) && return; for x in c` but instead have to call `iterate(c)` manually.

base/essentials.jl

@@ -775,6 +775,9 @@ inferencebarrier(@nospecialize(x)) = Ref{Any}(x)[]
 
 Determine whether a collection is empty (has no elements).
 
+This operation is guaranteed not to consume elements even for stateful
+iterators (which should define [`Base.isdone`](@ref) to that end).
+
 # Examples
 ```jldoctest
 julia> isempty([])