files: better doc for CreateFile and mark parameter unused

[kevinburkesegment]

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Describe the change

The first time I read this I wasn't sure how exactly the API was
receiving the file contents, so I added extra documentation to make
this more clear.

Tests
Shouldn't need tests as these are only documentation changes.

Additional context
Add any other context or screenshots or logs about your pull request here. If the pull request relates to an open issue, please link to it.

Issue: #XXXX

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.01%. Comparing base (774fc9d) to head (8f2dd32).
Report is 45 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #835      +/-   ##
==========================================
+ Coverage   98.46%   99.01%   +0.55%     
==========================================
  Files          24       26       +2     
  Lines        1364     1418      +54     
==========================================
+ Hits         1343     1404      +61     
+ Misses         15        8       -7     
  Partials        6        6              

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