gh-117521: Improve typing.TypeGuard docstring (#117522)

Lib/typing.py

@@ -843,20 +843,23 @@ def TypeGuard(self, parameters):
 
     For example::
 
-           def is_str(val: Union[str, float]):
-               # "isinstance" type guard
-               if isinstance(val, str):
-                   # Type of ``val`` is narrowed to ``str``
-                   ...
+         def is_str_list(val: list[object]) -> TypeGuard[list[str]]:
+             '''Determines whether all objects in the list are strings'''
+             return all(isinstance(x, str) for x in val)
+
+         def func1(val: list[object]):
+             if is_str_list(val):
+                 # Type of ``val`` is narrowed to ``list[str]``.
+                 print(" ".join(val))
              else:
-                   # Else, type of ``val`` is narrowed to ``float``.
-                   ...
+                 # Type of ``val`` remains as ``list[object]``.
+                 print("Not a list of strings!")
 
     Strict type narrowing is not enforced -- ``TypeB`` need not be a narrower
     form of ``TypeA`` (it can even be a wider form) and this may lead to
     type-unsafe results.  The main reason is to allow for things like
-    narrowing ``List[object]`` to ``List[str]`` even though the latter is not
-    a subtype of the former, since ``List`` is invariant.  The responsibility of
+    narrowing ``list[object]`` to ``list[str]`` even though the latter is not
+    a subtype of the former, since ``list`` is invariant.  The responsibility of
     writing type-safe type guards is left to the user.
 
     ``TypeGuard`` also works with type variables.  For more information, see