drm/xe: Add missing kernel docs in xe_exec_queue.c

drm/xe: Add missing kernel docs in xe_exec_queue.c

Add kernel doc to all exported functions that do not have
a kernel doc in xe_exec_queue.c.

v2: mention multi-lrc in comment (Matt Brost)

Assisted-by: Claude 4.5 Sonnet
Signed-off-by: Niranjana Vishwanathapura <niranjana.vishwanathapura@intel.com>
Reviewed-by: Matthew Brost <matthew.brost@intel.com>
Link: https://patch.msgid.link/20260303223040.140504-2-niranjana.vishwanathapura@intel.com
Link: https://patch.msgid.link/20260303223040.140504-2-niranjana.vishwanathapura@intel.com