Add documentation and raise exception when registering async reply function in sync chat (#1208)

* documentation update and added tests for register_reply function

* added raising an exception on an async reply function in sync chat

* big fixing

* test expanded

* Update autogen/agentchat/conversable_agent.py

Co-authored-by: Chi Wang <wang.chi@microsoft.com>

* Update autogen/agentchat/conversable_agent.py

Co-authored-by: Chi Wang <wang.chi@microsoft.com>

* refactorization

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Co-authored-by: Chi Wang <wang.chi@microsoft.com>