tty: Document defunct ASYNC_* bits in uapi header

Note the serial_struct flags for which the kernel ignores and performs
no action. The flags cannot be removed since they form part of the
userspace interface via the TIOCSSERIAL/TIOCGSERIAL ioctls.

Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

include/uapi/linux/tty_flags.h

@@ -8,6 +8,7 @@
  * Define ASYNCB_* for convenient use with {test,set,clear}_bit.
  *
  * Bits [0..ASYNCB_LAST_USER] are userspace defined/visible/changeable
+ * [x] in the bit comments indicates the flag is defunct and no longer used.
  */
 #define ASYNCB_HUP_NOTIFY	 0 /* Notify getty on hangups and closes
 				    * on the callout port */
@@ -19,15 +20,15 @@
 #define ASYNCB_SKIP_TEST	 6 /* Skip UART test during autoconfiguration */
 #define ASYNCB_AUTO_IRQ		 7 /* Do automatic IRQ during
 				    * autoconfiguration */
-#define ASYNCB_SESSION_LOCKOUT	 8 /* Lock out cua opens based on session */
-#define ASYNCB_PGRP_LOCKOUT	 9 /* Lock out cua opens based on pgrp */
-#define ASYNCB_CALLOUT_NOHUP	10 /* Don't do hangups for cua device */
+#define ASYNCB_SESSION_LOCKOUT	 8 /* [x] Lock out cua opens based on session */
+#define ASYNCB_PGRP_LOCKOUT	 9 /* [x] Lock out cua opens based on pgrp */
+#define ASYNCB_CALLOUT_NOHUP	10 /* [x] Don't do hangups for cua device */
 #define ASYNCB_HARDPPS_CD	11 /* Call hardpps when CD goes high  */
 #define ASYNCB_SPD_SHI		12 /* Use 230400 instead of 38400 bps */
 #define ASYNCB_LOW_LATENCY	13 /* Request low latency behaviour */
 #define ASYNCB_BUGGY_UART	14 /* This is a buggy UART, skip some safety
 				    * checks.  Note: can be dangerous! */
-#define ASYNCB_AUTOPROBE	15 /* Port was autoprobed by PCI or PNP code */
+#define ASYNCB_AUTOPROBE	15 /* [x] Port was autoprobed by PCI/PNP code */
 #define ASYNCB_MAGIC_MULTIPLIER	16 /* Use special CLK or divisor */
 #define ASYNCB_LAST_USER	16