doc: clarify stream 'close' event

doc/api/stream.markdown

@@ -42,9 +42,8 @@ Emitted if there was an error receiving data.
 
 `function () { }`
 
-Emitted when the underlying file descriptor has been closed. Not all streams
-will emit this.  (For example, an incoming HTTP request will not emit
-`'close'`.)
+Emitted when the underlying resource (for example, the backing file descriptor)
+has been closed. Not all streams will emit this.
 
 ### stream.readable