doc: add warning about stateful iterators to isempty (#43900)

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

@@ -773,6 +773,13 @@ inferencebarrier(@nospecialize(x)) = Ref{Any}(x)[]
 
 Determine whether a collection is empty (has no elements).
 
+!!! warning
+
+ `isempty(itr)` may consume the next element of a stateful iterator `itr`
+ unless an appropriate `Base.isdone(itr)` or `isempty` method is defined.
+ Use of `isempty` should therefore be avoided when writing generic
+ code which should support any iterator type.
+
 # Examples
 ```jldoctest
 julia> isempty([])