doc: add note about child process line buffering

Fixes #4808.

doc/api/child_process.markdown

@@ -6,7 +6,9 @@ Node provides a tri-directional `popen(3)` facility through the
 `child_process` module.
 
 It is possible to stream data through a child's `stdin`, `stdout`, and
-`stderr` in a fully non-blocking way.
+`stderr` in a fully non-blocking way.  (Note that some programs use
+line-buffered I/O internally.  That doesn't affect node.js but it means
+data you send to the child process is not immediately consumed.)
 
 To create a child process use `require('child_process').spawn()` or
 `require('child_process').fork()`.  The semantics of each are slightly