Revise exception handling doc on error handler functions

Document that a sync error-handler function is run in a threadpool.
Extend the add_error_handler function's Pydoc.

Author: chrisinmtown

connexion/middleware/main.py

@@ -452,9 +452,8 @@ def add_error_handler(
 
         :param code_or_exception: An exception class or the status code of HTTP exceptions to
             handle.
-        :param function: Callable that will handle exception and return a HTTP problem response.
-            An async coroutine has access to the stack traceback.  A sync function has no access
-            to the stack traceback because it is run in a threadpool.
+        :param function: Callable that will handle the exception and return a ConnexionResponse such
+            as a `connexion.problem.problem`. May be async; a sync function is run in a threadpool.
         """
         if self.middleware_stack is not None:
             raise RuntimeError(

docs/exceptions.rst

@@ -115,11 +115,8 @@ You can register error handlers on:
 
 .. note::
 
-    All the error handlers shown above are sync functions. The exception stack trace is
-    not available in a sync function because the middleware runs it in a threadpool.
-
-    Error handlers can be ``async`` coroutines as well.  Use a coroutine if the handler
-    function needs the stack trace from the exception, for example to log a traceback. 
+    Error handlers can be ``async`` coroutines as well. If the error handler needs to log
+    the exception, pass the exception using the Python logger's ``exc_info`` parameter.
 
 .. _Flask documentation: https://flask.palletsprojects.com/en/latest/errorhandling/#error-handlers