docs: clarify writableNeedDrain behavior and usage in streams

Author: oussamaBGZ

doc/api/stream.md

@@ -964,6 +964,7 @@ added:
 * {boolean}
 
 Is `true` if the stream's buffer has been full and stream will emit `'drain'`.
+This flag does not update immediately after each write and remains `true` until the writable stream drains and the `drain` event is emitted.
 
 ##### `writable.writableObjectMode`