doc: describe spawn option.detached

PR-URL: #2903
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>

doc/api/child_process.markdown

@@ -366,7 +366,8 @@ callback or returning an EventEmitter).
   * `env` {Object} Environment key-value pairs
   * `stdio` {Array|String} Child's stdio configuration. (See
     [below](#child_process_options_stdio))
-  * `detached` {Boolean} The child will be a process group leader.  (See
+  * `detached` {Boolean} Prepare child to run independently of its parent
+    process. Specific behavior depends on the platform, see
     [below](#child_process_options_detached))
   * `uid` {Number} Sets the user identity of the process. (See setuid(2).)
   * `gid` {Number} Sets the group identity of the process. (See setgid(2).)
@@ -505,9 +506,14 @@ Example:
 
 #### options.detached
 
-If the `detached` option is set, the child process will be made the leader of a
-new process group.  This makes it possible for the child to continue running
-after the parent exits.
+On Windows, this makes it possible for the child to continue running after the
+parent exits. The child will have a new console window (this cannot be
+disabled).
+
+On non-Windows, if the `detached` option is set, the child process will be made
+the leader of a new process group and session. Note that child processes may
+continue running after the parent exits whether they are detached or not.  See
+`setsid(2)` for more information.
 
 By default, the parent will wait for the detached child to exit.  To prevent
 the parent from waiting for a given `child`, use the `child.unref()` method,