BFU: Add comments in enum widget_type

I was trying to check which charset WIDGET_TEXT uses in the buffer but
had difficulty finding the code that handles key presses and updates
the buffer.  These comments should make that easier in the future.

src/bfu/common.h

@@ -13,13 +13,36 @@ typedef enum {
 /* Handler type for widgets. */
 typedef widget_handler_status_T (widget_handler_T)(struct dialog_data *, struct widget_data *);
 
-/* Type of widgets. */
+/** Type of widget.
+ *
+ * Each type has a struct widget_ops that contains pointers to
+ * type-specific handler functions.  For example, field_ops.kbd points
+ * to a function that handles keyboard events for WIDGET_FIELD.  */
 enum widget_type {
+	/** A check box or a radio button.
+	 * @see checkbox_ops */
 	WIDGET_CHECKBOX,
+
+	/** A single-line input field.
+	 * @see field_ops */
 	WIDGET_FIELD,
+
+	/** A single-line input field for a password.
+	 * Like #WIDGET_FIELD but shows asterisks instead of the text.
+	 * @see field_pass_ops */
 	WIDGET_FIELD_PASS,
+
+	/** A button that the user can push.
+	 * @see button_ops */
 	WIDGET_BUTTON,
+
+	/** A list or tree of items.
+	 * @see listbox_ops */
 	WIDGET_LISTBOX,
+
+	/** A block of text that the user cannot edit but may be able
+	 * to scroll.
+	 * @see text_ops */
 	WIDGET_TEXT,
 };
 

src/bfu/widget.h

@@ -61,7 +61,9 @@ struct widget_data {
 	 * that points to the first element of an array.  Each element
 	 * in this array corresponds to one line of text, and is an
 	 * unsigned char * that points to the first character of that
-	 * line.  The array has @widget_data.info.text.lines elements.  */
+	 * line.  The array has @widget_data.info.text.lines elements.
+	 *
+	 * For WIDGET_LISTBOX: @cdata points to struct listbox_data.  */
 	unsigned char *cdata;
 
 	struct box box;