Tidy up the notes in the stop_animation docstrings (#3019)

In one case there was a hangover from before we added the `complete` option;
in the other two cases it also needed to be made clear that it's only a
no-op if there's no animation running or also scheduled.

Author: davep

src/textual/_animator.py

@@ -476,9 +476,7 @@ async def stop_animation(
             complete: Should the animation be set to its final value?
 
         Note:
-            If there is no animation running, this is a no-op. If there is
-            an animation running the attribute will be left in the last
-            state it was in before the call to stop.
+            If there is no animation scheduled or running, this is a no-op.
         """
         key = (id(obj), attribute)
         if key in self._scheduled:

src/textual/app.py

@@ -609,7 +609,7 @@ async def stop_animation(self, attribute: str, complete: bool = True) -> None:
             complete: Should the animation be set to its final value?
 
         Note:
-            If there is no animation running, this is a no-op.
+            If there is no animation scheduled or running, this is a no-op.
         """
         await self._animator.stop_animation(self, attribute, complete)
 

src/textual/widget.py

@@ -1575,7 +1575,7 @@ async def stop_animation(self, attribute: str, complete: bool = True) -> None:
             complete: Should the animation be set to its final value?
 
         Note:
-            If there is no animation running, this is a no-op.
+            If there is no animation scheduled or running, this is a no-op.
         """
         await self.app.animator.stop_animation(self, attribute, complete)