update set_custom_exc docstring with new-style Parameters section

IPython/core/interactiveshell.py

@@ -1447,30 +1447,38 b' class InteractiveShell(SingletonConfigurable, Magic):'
 
         Set a custom exception handler, which will be called if any of the
         exceptions in exc_tuple occur in the mainloop (specifically, in the
-        run_code() method.
+        run_code() method).
 
-        Inputs:
+        Parameters
+        ----------
 
-          - exc_tuple: a *tuple* of valid exceptions to call the defined
-          handler for.  It is very important that you use a tuple, and NOT A
+        exc_tuple : tuple of exception classes
+            A *tuple* of exception classes, for which to call the defined
+            handler.  It is very important that you use a tuple, and NOT A
           LIST here, because of the way Python's except statement works.  If
-          you only want to trap a single exception, use a singleton tuple:
+            you only want to trap a single exception, use a singleton tuple::
 
             exc_tuple == (MyCustomException,)
 
-          - handler: this must be defined as a function with the following
-          basic interface::
+        handler : callable
+            handler must have the following signature::
 
-            def my_handler(self, etype, value, tb, tb_offset=None)
+                def my_handler(self, etype, value, tb, tb_offset=None):
                 ...
-                # The return value must be
                 return structured_traceback
 
+            Your handler must return a structured traceback (a list of strings),
+            or None.
+
           This will be made into an instance method (via types.MethodType)
           of IPython itself, and it will be called if any of the exceptions
           listed in the exc_tuple are caught.  If the handler is None, an
           internal basic one is used, which just prints basic info.
 
+            To protect IPython from crashes, if your handler ever raises an
+            exception or returns an invalid result, it will be immediately
+            disabled.
+
         WARNING: by putting in your own exception handler into IPython's main
         execution loop, you run a very good chance of nasty crashes.  This
         facility should only be used if you really know what you are doing."""