Change fail_after&move_on_after to set deadline relative to entering. Add CancelScope.relative_deadline

src/trio/_timeouts.py

@@ -25,7 +25,7 @@ def move_on_at(deadline: float) -> trio.CancelScope:
 
 def move_on_after(seconds: float) -> trio.CancelScope:
     """Use as a context manager to create a cancel scope whose deadline is
-    set to now + *seconds*.
+    set to creation time + *seconds*.
 
     Args:
       seconds (float): The timeout.
@@ -138,6 +138,9 @@ def fail_after(seconds: float) -> AbstractContextManager[trio.CancelScope]:
     it's caught and discarded. When it reaches :func:`fail_after`, then it's
     caught and :exc:`TooSlowError` is raised in its place.
 
+    The deadline of the cancel scope is calculated at creation time, not upon
+    entering the context manager.
+
     Args:
       seconds (float): The timeout.