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.

Fixes #2019

Author: chrisinmtown

connexion/middleware/main.py

@@ -452,7 +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, may be async.
+        :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

@@ -62,7 +62,7 @@ You can register error handlers on:
 
             .. warning::
 
-                ⚠️ **The following is not recommended as it complicates the exception handling logic,**
+                ⚠️ **The following is not recommended as it complicates the exception handling logic!**
 
             You can also register error handlers on the underlying flask application directly.
 
@@ -115,7 +115,8 @@ You can register error handlers on:
 
 .. note::
 
-    Error handlers can be ``async`` coroutines as well.
+    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