Add documentation and raise exception when registering async reply function in sync chat

[davorrunje]

Why are these changes needed?

Current state:

1. If you register a sync reply function and initiate an async chat, the reply function will be triggered.
2. If you register an async reply function and initiate a sync chat, the reply function will not be triggered.
3. and 4. if the reply function and chat initiation are either both sync or both async, then the reply function would be triggered.

Proposed change:

2. If you register an async reply function and initiate a sync chat, a RunTime exception will be raised unless we explicitly specify that the function should be ignored in sync chat.

Related issue number

Closes #1195

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 (2e519b0) 30.63% compared to head (d548a29) 50.30%.

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #1208       +/-   ##
===========================================
+ Coverage   30.63%   50.30%   +19.67%     
===========================================
  Files          32       32               
  Lines        4365     4381       +16     
  Branches     1015     1077       +62     
===========================================
+ Hits         1337     2204      +867     
+ Misses       2932     1964      -968     
- Partials       96      213      +117     

Flag	Coverage Δ
unittests	50.21% <100.00%> (+19.63%)	⬆️

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