Add a comment to explain the role of UNSTRUCTURED_FORMATS

[thinkall]

Why are these changes needed?

Address #891

Related issue number

Closes #891

Checks

• I've included any doc changes needed for https://microsoft.github.io/autogen/. See https://microsoft.github.io/autogen/docs/Contribute#documentation to build and test documentation locally.
• I've added tests (if relevant) corresponding to the changes introduced in this PR.
• I've made sure all auto checks have passed.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (1b4fb8f) 26.58% compared to head (7c49000) 30.68%.

❗ Current head 7c49000 differs from pull request most recent head 7855c4b. Consider uploading reports for the commit 7855c4b to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #909      +/-   ##
==========================================
+ Coverage   26.58%   30.68%   +4.09%     
==========================================
  Files          28       28              
  Lines        3732     3732              
  Branches      847      847              
==========================================
+ Hits          992     1145     +153     
+ Misses       2667     2502     -165     
- Partials       73       85      +12     

Flag	Coverage Δ
unittests	30.68% <ø> (+4.15%)	⬆️

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[thinkall]

@szwang-seedtech, does the comment added in this PR make the code clearer for you?