fetch is a noop when on the same worker (JuliaLang#19044)

I was quite confused by this example, since I didn't realize that there is a `getindex(f::Future, args...) = getindex(fetch(f), args...)`.
Because of this I misinterpreted the example (my first conclusion: `remote_call` must magically unpack the `Future`).
Also, it didn't really make clear, that fetch turns into a noop if applied to a `Future` from the same worker.
I tried to add this information in a concise way.

doc/src/manual/distributed-computing.md

@@ -80,10 +80,18 @@ you read from a remote object to obtain data needed by the next local operation.
 but is more efficient.
 
 ```julia-repl
-julia> remotecall_fetch(getindex, 2, r, 1, 1)
+julia> remotecall_fetch(r-> fetch(r)[1, 1], 2, r)
 0.18526337335308085
 ```
 
+This fetches the array on worker 2 and returns the first value. Note, that `fetch` doesn't move any data in
+this case, since it's executed on the worker that owns the array. One can also write:
+
+```julia-repl
+julia> remotecall_fetch(getindex, 2, r, 1, 1)
+0.10824216411304866
+```
+
 Remember that [`getindex(r,1,1)`](@ref) is [equivalent](@ref man-array-indexing) to `r[1,1]`, so this call fetches
 the first element of the future `r`.