docs: ABI: testing: Document the SBEFIFO timeout interface

Add documentation for the new sysfs entry that indicates whether or not
the SBE has timed out.
Signed-off-by: Eddie James <eajames@linux.ibm.com>
Link: https://lore.kernel.org/r/20211019211749.38059-2-eajames@linux.ibm.comSigned-off-by: Joel Stanley <joel@jms.id.au>

Documentation/ABI/testing/sysfs-bus-fsi-devices-sbefifo

+What:		/sys/bus/fsi/devices/XX.XX.00:06/sbefifoX/timeout
+KernelVersion:	5.15
+Contact:	eajames@linux.ibm.com
+Description:
+		Indicates whether or not this SBE device has experienced a
+		timeout; i.e. the SBE did not respond within the time allotted
+		by the driver. A value of 1 indicates that a timeout has
+		ocurred and no transfers have completed since the timeout. A
+		value of 0 indicates that no timeout has ocurred, or if one
+		has, more recent transfers have completed successful.