drm/xe/configfs: Improve doc for ctx_restore* attributes

Spell out the syntax instead of only using examples. Particularly
important the <engine-class> part since that's different than
engines_allowed and may confuse users. The same batch buffer is used for
all engines of a certain class.

Cc: Raag Jadav <raag.jadav@intel.com>
Reviewed-by: Raag Jadav <raag.jadav@intel.com>
Reviewed-by: Jonathan Cavitt <jonathan.cavitt@intel.com>
Fixes: e2a9854d80 ("drm/xe/configfs: Allow to select by class only")
Link: https://lore.kernel.org/r/20250924152709.659483-4-lucas.demarchi@intel.com
Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
(cherry picked from commit 47ca7acff4)
Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>

drivers/gpu/drm/xe/xe_configfs.c

@@ -126,8 +126,20 @@
  * not intended for normal execution and will taint the kernel with TAINT_TEST
  * when used.
  *
- * Currently this is implemented only for post and mid context restore.
- * Examples:
+ * The syntax allows to pass straight instructions to be executed by the engine
+ * in a batch buffer or set specific registers.
+ *
+ * #. Generic instruction::
+ *
+ *	<engine-class> cmd <instr> [[dword0] [dword1] [...]]
+ *
+ * #. Simple register setting::
+ *
+ *	<engine-class> reg <address> <value>
+ *
+ * Commands are saved per engine class: all instances of that class will execute
+ * those commands during context switch. The instruction, dword arguments,
+ * addresses and values are in hex format like in the examples below.
  *
  * #. Execute a LRI command to write 0xDEADBEEF to register 0x4f10 after the
  *    normal context restore::
@@ -154,7 +166,8 @@
  *       When using multiple lines, make sure to use a command that is
  *       implemented with a single write syscall, like HEREDOC.
  *
- * These attributes can only be set before binding to the device.
+ * Currently this is implemented only for post and mid context restore and
+ * these attributes can only be set before binding to the device.
  *
  * Remove devices
  * ==============