scsi: documentation: Document Fibre Channel sysfs node for appid

Update documentation for sysfs node within /sys/class/fc/fc_udev_device/.

Link: https://lore.kernel.org/r/20210913015853.2086512-1-muneendra.kumar@broadcom.com
Reviewed-by: Himanshu Madhani <himanshu.madhani@oracle.com>
Signed-off-by: Muneendra Kumar <muneendra.kumar@broadcom.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>

diff --git a/Documentation/ABI/testing/sysfs-class-fc b/Documentation/ABI/testing/sysfs-class-fc
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+What:          /sys/class/fc/fc_udev_device/appid_store
+Date:          Aug 2021
+Contact:       Muneendra Kumar <muneendra.kumar@broadconm.com>
+Description:
+               This interface allows an admin to set an FC application
+               identifier in the blkcg associated with a cgroup id. The
+               identifier is typically a UUID that is associated with
+               an application or logical entity such as a virtual
+               machine or container group. The application or logical
+               entity utilizes a block device via the cgroup id.
+               FC adapter drivers may query the identifier and tag FC
+               traffic based on the identifier. FC host and FC fabric
+               entities can utilize the application id and FC traffic
+               tag to identify traffic sources.
+
+               The interface expects a string "<cgroupid>:<appid>" where:
+               <cgroupid> is inode of the cgroup in hexadecimal
+               <appid> is user provided string upto 128 characters
+               in length.
+
+               If an appid_store is done for a cgroup id that already
+               has an appid set, the new value will override the
+               previous value.
+
+               If an admin wants to remove an FC application identifier
+               from a cgroup, an appid_store should be done with the
+               following string: "<cgroupid>:"