Fix FileBuffer usage documentation

[dreikanter]

Changes:

• Update the FileBuffer examples to pass the source URL or path to load.
• Remove the stale constructor parameter documentation.

Why:

The documented constructor API no longer matches the implementation.

References:

• Related issue: Review the code structure #1010 (F-122)

Checks:

• Executable code is unchanged.
• GitHub Actions will verify the branch.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.55%. Comparing base (ce0547a) to head (dc0ac23).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1090   +/-   ##
=======================================
  Coverage   97.55%   97.55%           
=======================================
  Files         355      355           
  Lines        8358     8358           
=======================================
  Hits         8154     8154           
  Misses        204      204           

Files with missing lines	Coverage Δ
app/services/file_buffer.rb	100.00% <ø> (ø)

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