clarify the behavior of newChan

follow up the advice of `timothee`

lib/std/channels.nim

@@ -506,5 +506,9 @@ func close*[T](c: Channel[T]): bool {.inline.} =
 func peek*[T](c: Channel[T]): int {.inline.} = peek(c.d)
 
 proc newChannel*[T](elements = 30): Channel[T] =
+  ## Returns a new `Channel`. `elements` should be positive.
+  ## `elements` is used to specify whether a channel is buffered or not.
+  ## If `elements` = 1, the channel is unbuffered. If `elements` > 1, the 
+  ## channel is buffered.
   assert elements >= 1, "Elements must be positive!"
   result = Channel[T](d: allocChannel(sizeof(T), elements))