This capability was added in May 2014, and the previous Issue 2475 has been closed.
The examples given in Google's Documentation are OK, but you can do more with the jsdoc information than they show.
For example:
/**
* Return the time that the referenced cell or range was last changed.
* Initial use shows current time.
*
* @param {Sheet3!B32} reference Cell or range to monitor.
* @returns The time the reference was last changed.
* @customfunction
*/
function lastModified( reference ) {
// Surprise - we don't actually care what has changed!
return( new Date() );
}
I covered this with more detail and examples in my blog.