doc: document that stdio is usually blocking

doc/api/stdio.markdown

@@ -9,6 +9,19 @@
 For printing to stdout and stderr.  Similar to the console object functions
 provided by most web browsers, here the output is sent to stdout or stderr.
 
+The console functions are synchronous when the destination is a terminal or
+a file (to avoid lost messages in case of premature exit) and asynchronous
+when it's a pipe (to avoid blocking for long periods of time).
+
+That is, in the following example, stdout is non-blocking while stderr
+is blocking:
+
+    $ node script.js 2> error.log | tee info.log
+
+In daily use, the blocking/non-blocking dichotomy is not something you
+should worry about unless you log huge amounts of data.
+
+
 ## console.log([data], [...])
 
 Prints to stdout with newline. This function can take multiple arguments in a