bitbang: document bitbang callbacks

Change-Id: I732c2eeb452f3ba8a2385d0e02fccbe86381812c
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: http://openocd.zylin.com/5553
Tested-by: jenkins

src/jtag/drivers/bitbang.h

@@ -38,21 +38,29 @@ typedef enum {
  * sample requests together. Not waiting for a value to come back can greatly
  * increase throughput. */
 struct bitbang_interface {
-	/** Sample TDO. */
+	/** Sample TDO and return the value. */
 	bb_value_t (*read)(void);
 
 	/** The number of TDO samples that can be buffered up before the caller has
 	 * to call read_sample. */
 	size_t buf_size;
+
 	/** Sample TDO and put the result in a buffer. */
 	int (*sample)(void);
+
 	/** Return the next unread value from the buffer. */
 	bb_value_t (*read_sample)(void);
 
 	/** Set TCK, TMS, and TDI to the given values. */
 	int (*write)(int tck, int tms, int tdi);
+
+	/** Blink led (optional). */
 	int (*blink)(int on);
+
+	/** Sample SWDIO and return the value. */
 	int (*swdio_read)(void);
+
+	/** Set direction of SWDIO. */
 	void (*swdio_drive)(bool on);
 };