doc: concurency example with WaitGroup (#6034)

doc/docs.md

@@ -1683,6 +1683,36 @@ V's model of concurrency is very similar to Go's. To run `foo()` concurrently, j
 call it with `go foo()`. Right now, it launches the function on a new system
 thread. Soon coroutines and a scheduler will be implemented.
 
+```v
+import sync
+import time
+
+fn task(id, duration int, mut wg sync.WaitGroup) {
+    println("task ${id} begin")
+    time.sleep_ms(duration)
+    println("task ${id} end")
+    wg.done()
+}
+
+fn main() {
+    mut wg := sync.new_waitgroup()
+    wg.add(3)
+    go task(1, 500, mut wg)
+    go task(2, 900, mut wg)
+    go task(3, 100, mut wg)
+    wg.wait()
+    println('done')
+}
+
+// Output: task 1 begin
+//         task 2 begin
+//         task 3 begin
+//         task 3 end
+//         task 1 end
+//         task 2 end
+//         done
+```
+
 Unlike Go, V has no channels (yet). Nevertheless, data can be exchanged between a coroutine
 and the calling thread via a shared variable. This variable should be created as reference and passed to
 the coroutine as `mut`. The underlying `struct` should also contain a `mutex` to lock concurrent access: