It's not great but if you right-click a text field for example - when it's not protected you can select 'Properties'
In the Properties dialog box there is a button - "Add Help Text..."
Click this and you can add some custom help text.
When the form is protected, the help text shows in the information bar at the bottom of the Word window - it's not very obvious - but it's better than nothing.
You can also attach macros to form fields. I did one programatically - so that a message box pops up when the field is entered.
ActiveDocument.FormFields(1).EntryMacro = "MyTest"
After running the ffMessage macro, the first form field responded with the message box when I entered the field.
This might be something you could explore further.