add relative_deadline docstring

docs/source/conf.py

@@ -171,7 +171,6 @@ def autodoc_process_signature(
 # currently undocumented things
 logger = getLogger("trio")
 UNDOCUMENTED = {
-    "trio.CancelScope.relative_deadline",
     "trio.MemorySendChannel",
     "trio.MemoryReceiveChannel",
     "trio.MemoryChannelStatistics",
@@ -201,7 +200,9 @@ def autodoc_process_docstring(
         logger.warning(f"{name} has no docstring")
     else:
         if name in UNDOCUMENTED:
-            logger.warning("outdated list of undocumented things")
+            logger.warning(
+                f"outdated list of undocumented things in docs/source/conf.py: {name!r} has a docstring"
+            )
 
 
 def setup(app: Sphinx) -> None:

src/trio/_core/_run.py

@@ -802,6 +802,18 @@ def deadline(self, new_deadline: float) -> None:
 
     @property
     def relative_deadline(self) -> float:
+        """Read-write, :class:`float`. The number of seconds remaining until this
+        scope's deadline, relative to the current time.
+
+        Defaults to :data:`math.inf` ("no deadline"). Must be non-negative.
+
+        When modified
+        Before entering: sets the deadline relative to when the scope enters.
+        After entering: sets a new deadline relative to the current time.
+
+        Raises:
+          RuntimeError: if trying to read or modify an unentered scope with an absolute deadline, i.e. when :attr:`is_relative` is ``False``.
+        """
         if self._has_been_entered:
             return self._deadline - current_time()
         elif self._deadline != inf: