Kernel: Add a comment block about the Device class.

Kernel/Devices/Device.h

@@ -1,5 +1,38 @@
 #pragma once
 
+// Device is the base class of everything that lives in the /dev directory.
+//
+// All Devices will automatically register with the VFS.
+// To expose a Device to the filesystem, simply pass two unique numbers to the constructor,
+// and then mknod a file in /dev with those numbers.
+//
+// There are two main subclasses:
+//   - BlockDevice (random access)
+//   - CharacterDevice (sequential)
+//
+// The most important functions in Device are:
+//
+// class_name()
+//   - Used in the /proc filesystem to identify the type of Device.
+//
+// read() and write()
+//   - Implement reading and writing.
+//   - Return the number of bytes read/written, OR a negative error code.
+//
+// can_read() and can_write()
+//
+//   - Used to implement blocking I/O, and the select() and poll() syscalls.
+//   - Return true if read() or write() would succeed, respectively.
+//   - Note that can_read() should return true in EOF conditions,
+//     and a subsequent call to read() should return 0.
+//
+// ioctl()
+//
+//   - Optional. If unimplemented, ioctl() on the device will fail with -ENOTTY.
+//   - Can be overridden in subclasses to implement arbitrary functionality.
+//   - Subclasses should take care to validate incoming addresses before dereferencing.
+//
+
 #include <AK/Retainable.h>
 #include <AK/Types.h>
 #include <Kernel/FileSystem/FileDescriptor.h>