Sub lable on input field

Many times we find we need to add an explanation on an input field and its too long to write on the label itself. Placeholder isn’t a good solutions as it disappears when they start filling the field in.
What we do is hide the label and asterisk and add two captions to hold the label+asterisk and sub label (and wrap all three in an autofit container). 
So, as you see it would be really nice if you had a build in option to add a sub label.