doc: fix a few pytest.mark.xfail nits

Refs #9027, #10094.

src/_pytest/mark/structures.py

@@ -461,7 +461,9 @@ if TYPE_CHECKING:
             *conditions: Union[str, bool],
             reason: str = ...,
             run: bool = ...,
-            raises: Union[Type[BaseException], Tuple[Type[BaseException], ...]] = ...,
+            raises: Union[
+                None, Type[BaseException], Tuple[Type[BaseException], ...]
+            ] = ...,
             strict: bool = ...,
         ) -> MarkDecorator:
             ...

src/_pytest/outcomes.py

@@ -233,6 +233,9 @@ def xfail(reason: str = "") -> NoReturn:
 
     This function should be called only during testing (setup, call or teardown).
 
+    No other code is executed after using ``xfail()`` (it is implemented
+    internally by raising an exception).
+
     :param reason:
         The message to show the user as reason for the xfail.