sync/channels: provide .cap and .len() (#6104)

doc/upcoming.md

@@ -250,6 +250,15 @@ if ch.pop(&m) {
 }
 ```
 
+There are also methods `try_push()` and `try_pop()` which return immediatelly with the return value `.not_ready` if the transaction
+cannot be performed without waiting. The return value is of type `sync.TransactionState` which can also be
+`.success` or `.closed`.
+
+To monitor a channel there is a method `len()` which returns the number of elements currently in the queue and the attribute
+`cap` for the queue length. Please be aware that in general `channel.len() > 0` does not guarantee that the next
+`pop()` will succeed without waiting, since other threads may already have "stolen" elements from the queue. Use `try_pop()` to
+accomplish this kind of task.
+
 The select call is somewhat tricky. The `channel_select()` function needs three arrays that
 contain the channels, the directions (pop/push) and the object references and
 a timeout of type `time.Duration` (`time.infinite` or `-1` to wait unlimited) as parameters. It returns the