Minor code documentation improvements

[adrg]

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[codecov]

Codecov Report

Merging #352 into v3-a5i-pdf-sign will decrease coverage by 5.37%.
The diff coverage is 50%.

@@                 Coverage Diff                 @@
##           v3-a5i-pdf-sign     #352      +/-   ##
===================================================
- Coverage            60.19%   54.82%   -5.38%     
===================================================
  Files                  152      152              
  Lines                27222    27161      -61     
===================================================
- Hits                 16386    14890    -1496     
+ Misses               10447    10432      -15     
- Partials               389     1839    +1450

Impacted Files	Coverage Δ
pdf/model/sighandler/sighandler_rsa_sha1.go	5.4% <ø> (-1.36%)	⬇️
pdf/model/signature.go	77.86% <0%> (+18.42%)	⬆️
pdf/model/appender.go	76.48% <75%> (-12.71%)	⬇️
pdf/creator/utils.go	55.55% <0%> (-22.23%)	⬇️
pdf/core/security/handlers.go	33.33% <0%> (-16.67%)	⬇️
pdf/model/optimize/image_ppi.go	56.71% <0%> (-16.42%)	⬇️
pdf/model/reader.go	50.5% <0%> (-15.94%)	⬇️
pdf/ps/exec.go	62.96% <0%> (-14.82%)	⬇️
pdf/ps/operations.go	53.45% <0%> (-13.84%)	⬇️
pdf/model/signature_handler.go	55.04% <0%> (-13.77%)	⬇️
... and 95 more

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