jtag: report API reset as synchronous

The jtag API reset() is synchronous, but this was not highlighted
in the description.

Change-Id: I76ffb7eec97c8608cfbef0b9268ee18a5f50b221
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Fixes: 8850eb8f2c ("swd: get rid of jtag queue to assert/deassert srst")
Reviewed-on: http://openocd.zylin.com/5471
Reviewed-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>
Tested-by: jenkins

src/jtag/interface.h

@@ -250,6 +250,8 @@ struct adapter_driver {
 
 	/**
 	 * Control (assert/deassert) the signals SRST and TRST on the interface.
+	 * This function is synchronous and should be called after the adapter
+	 * queue has been properly flushed.
 	 * This function is optional.
 	 * Adapters that don't support resets can either not define this function
 	 * or return an error code.