docs: improve usage guide for selective readers

[ST-DDT]

Some users (including me) tend to read only code blocks that contains actual code (and not data structures).
Thus it is easily missed that User is a custom/example data-structure that we defined ourselves.

This PR adds a tiny hint to the first code example, so it becomes clear that User is a example data-structure even if you read only that example.

I also considered adding the hint to all examples, but that code block is more or less the entry point so it should be sufficient to add it there.

Ref: Discord User Question

[codecov]

Codecov Report

Merging #1406 (0048de2) into main (a17872b) will not change coverage.
The diff coverage is n/a.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1406   +/-   ##
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  Coverage   99.62%   99.62%           
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  Files        2165     2165           
  Lines      241432   241432           
  Branches     1017     1017           
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  Hits       240525   240525           
  Misses        886      886           
  Partials       21       21           

[Shinigami92]

works 👌