videosink: Add missing comment on Display structure

[hansmi]

Relates to #26.

[codecov-commenter]

Codecov Report

Merging #31 (21a1056) into main (42ee855) will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff          @@
##            main     #31   +/-   ##
=====================================
  Coverage   61.2%   61.2%           
=====================================
  Files         53      53           
  Lines       6672    6672           
=====================================
  Hits        4085    4085           
  Misses      2446    2446           
  Partials     141     141           

Impacted Files	Coverage Δ
videosink/display.go	87.5% <ø> (ø)

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[maruel]

Thanks!