doc: Clarified read method with specified size argument.

Made explicitely clear that when size bytes are not available, it will
return null, unless we've ended, in which case the data remaining in the
buffer will be returned.

Fixes nodejs#7273

doc/api/stream.markdown

@@ -227,7 +227,9 @@ returns it.  If there is no data available, then it will return
 `null`.
 
 If you pass in a `size` argument, then it will return that many
-bytes.  If `size` bytes are not available, then it will return `null`.
+bytes.  If `size` bytes are not available, then it will return `null`,
+unless we've ended, in which case it will return the data remaining 
+in the buffer.
 
 If you do not specify a `size` argument, then it will return all the
 data in the internal buffer.