Hi RBM,
I don't have much time... But I now I used that once. Here it is.
Ex. on ad_form
{upload_file:file(file),optional {help_text {Only .txt files will be accepted.}} {label "or Upload a File"}}
On ad_form using {mywidget:text(text) {help_text {my help text}}} "my help text" will be placed under mywidget.
Sorry haven't used on non ad_form. Hopefully its an ok start.