drm: writeback: document callbacks

Document the callbacks:
    drm_connector_helper_funcs.prepare_writeback_job
    drm_connector_helper_funcs.cleanup_writeback_job

The documentation was pulled from the changelong introducing the
callbacks, originally written by Laurent.

Adding the missing documentation fixes the following warnings:
drm_modeset_helper_vtables.h:1052: warning: Function parameter or member 'prepare_writeback_job' not described in 'drm_connector_helper_funcs'
drm_modeset_helper_vtables.h:1052: warning: Function parameter or member 'cleanup_writeback_job' not described in 'drm_connector_helper_funcs'

v2:
  - Fix formatting (Daniel)
  - Drop changelog text and add reference (Daniel)
  - Improve grammar. and use "operation" (Laurent)

Signed-off-by: Sam Ravnborg <[email protected]>
Reviewed-by: Daniel Vetter <[email protected]>
Reviewed-by: Laurent Pinchart <[email protected]>
Reviewed-by: Liviu Dudau <[email protected]>
Cc: Laurent Pinchart <[email protected]>
Cc: Liviu Dudau <[email protected]>
Cc: Daniel Vetter <[email protected]>
Cc: Maarten Lankhorst <[email protected]>
Cc: Maxime Ripard <[email protected]>
Cc: Thomas Zimmermann <[email protected]>
Cc: David Airlie <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]

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