Add documentation and TODOs for xlang

[peterychang]

Why are these changes needed?

TODOs and clarifying documentation

Related issue number

#4363

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]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 68.35%. Comparing base (7b10f0a) to head (611dd14).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4926   +/-   ##
=======================================
  Coverage   68.34%   68.35%           
=======================================
  Files         157      157           
  Lines       10104    10104           
=======================================
+ Hits         6906     6907    +1     
+ Misses       3198     3197    -1     

Flag	Coverage Δ
unittests	68.35% <ø> (+<0.01%)	⬆️

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