doc: fix a few xfail nits

doc/en/reference/reference.rst

@@ -237,7 +237,7 @@ pytest.mark.xfail
 
 Marks a test function as *expected to fail*.
 
-.. py:function:: pytest.mark.xfail(condition=None, *, reason=None, raises=None, run=True, strict=False)
+.. py:function:: pytest.mark.xfail(condition=None, *, reason=None, raises=None, run=True, strict=xfail_strict)
 
     :type condition: bool or str
     :param condition:
@@ -249,17 +249,19 @@ Marks a test function as *expected to fail*.
     :keyword Type[Exception] raises:
         Exception subclass (or tuple of subclasses) expected to be raised by the test function; other exceptions will fail the test.
     :keyword bool run:
-        If the test function should actually be executed. If ``False``, the function will always xfail and will
+        Whether the test function should actually be executed. If ``False``, the function will always xfail and will
         not be executed (useful if a function is segfaulting).
     :keyword bool strict:
-        * If ``False`` (the default) the function will be shown in the terminal output as ``xfailed`` if it fails
+        * If ``False`` the function will be shown in the terminal output as ``xfailed`` if it fails
           and as ``xpass`` if it passes. In both cases this will not cause the test suite to fail as a whole. This
           is particularly useful to mark *flaky* tests (tests that fail at random) to be tackled later.
         * If ``True``, the function will be shown in the terminal output as ``xfailed`` if it fails, but if it
           unexpectedly passes then it will **fail** the test suite. This is particularly useful to mark functions
           that are always failing and there should be a clear indication if they unexpectedly start to pass (for example
           a new release of a library fixes a known bug).
 
+        Defaults to :confval:`xfail_strict`, which is ``False`` by default.
+
 
 Custom marks
 ~~~~~~~~~~~~

src/_pytest/mark/structures.py

@@ -461,7 +461,9 @@ def __call__(
             *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.