Document child_process.execFile

Fixes #1700

doc/api/child_processes.markdown

@@ -142,8 +142,7 @@ See also: `child_process.exec()`
 
 ### child_process.exec(command, [options], callback)
 
-High-level way to execute a command as a child process, buffer the
-output, and return it all in a callback.
+Runs a command in a shell and buffers the output.
 
     var util = require('util'),
         exec = require('child_process').exec,
@@ -179,6 +178,13 @@ amount of data allowed on stdout or stderr - if this value is exceeded then
 the child process is killed.
 
 
+### child_process.execFile(file, args, options, callback)
+
+This is similar to `child_process.exec()` except it does not execute a
+subshell but rather the specified file directly. This makes it slightly
+leaner than `child_process.exec`. It has the same options.
+
+
 ### child_process.fork(modulePath, arguments, options)
 
 This is a special case of the `spawn()` functionality for spawning Node