docs: filesystems: sysfs: add remaining top level sysfs directory descriptions

Finish top level sysfs directory descriptions for block, class, firmware,
hypervisor, kernel, and power. Did not write one for net directory. See
commit bc3a88431672 ("docs: filesystems: sysfs: remove top level sysfs net
 directory")

Signed-off-by: Alex Tran <alex.t.tran@gmail.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Message-ID: <20250902023039.1351270-1-alex.t.tran@gmail.com>

Author: alectric-tr

Documentation/filesystems/sysfs.rst

@@ -334,8 +334,22 @@ a stat(2) operation.
 More information on driver-model specific features can be found in
 Documentation/driver-api/driver-model/.
 
+block/ contains symlinks to all the block devices discovered on the system.
+These symlinks point to directories under /sys/devices.
 
-TODO: Finish this section.
+class/ contains a directory for each device class, grouped by functional type.
+Each directory in class/ contains symlinks to devices in the /sys/devices directory.
+
+firmware/ contains system firmware data and configuration such as firmware tables,
+ACPI information, and device tree data.
+
+hypervisor/ contains virtualization platform information and provides an interface to
+the underlying hypervisor.  It is only present when running on a virtual machine.
+
+kernel/ contains runtime kernel parameters, configuration settings, and status.
+
+power/ contains power management subsystem information including
+sleep states, suspend/resume capabilities, and policies.
 
 
 Current Interfaces