vlc_vout_display: fix doxygen

 - references to member from different structure without context:
    - window -> vout_display_cfg.window
    - is_display_filled -> vout_display_cfg.is_display_filled
    - reset_pictures -> vlc_display_operations.reset_pictures

 - fields that have been removed
    - cfg in display activation callback

 - invalid references: ::source