docs: add initial target rtos support info

Change-Id: Idd39ce17922602aedd4626496ed8f5422bb76e07
Signed-off-by: Spencer Oliver <spen@spen-soft.co.uk>
Reviewed-on: http://openocd.zylin.com/641
Tested-by: jenkins
Reviewed-by: Peter Stuge <peter@stuge.se>

doc/openocd.texi

@@ -3940,6 +3940,10 @@ base @var{address} to be used when an MMU is active.
 The value should normally correspond to a static mapping for the
 @code{-work-area-phys} address, set up by the current operating system.
 
+@item @code{-rtos} @var{rtos_type} -- enable rtos support for target,
+@var{rtos_type} can be one of @option{auto}|@option{eCos}|@option{threadx}|
+@option{freertos}|@option{linux}.
+
 @end itemize
 @end deffn