Adding context help to a form is very simple. Select the Properties tab of your field, enter the help text and select the Display context help option:
After that, an icon with a question mark is displayed next to the field. Mousing over the icon displays a bubble with your help text:
The question mark (the icon file) and the bubble (the font, background and borders) can both be changed with Theme Editor by selecting Context help icon and Context help layer in the tree.
In the case of Radiobutton list and Checkbox list fields, context help can be associated with specific options: