docs: update selection list messages docs (#4399)

* remove selection toggled note

* update selected changed docstring

* add event handler to selected changed docstring

* remove differentiation of user interaction and API

docs/widgets/selection_list.md

@@ -117,15 +117,8 @@ collection of selected values:
 
 ## Messages
 
-The following messages will be posted as the user interacts with the list:
-
 - [SelectionList.SelectionHighlighted][textual.widgets.SelectionList.SelectionHighlighted]
 - [SelectionList.SelectionToggled][textual.widgets.SelectionList.SelectionToggled]
-
-The following message will be posted if the content of
-[`selected`][textual.widgets.SelectionList.selected] changes, either by user
-interaction or by API calls:
-
 - [SelectionList.SelectedChanged][textual.widgets.SelectionList.SelectedChanged]
 
 ## Bindings

src/textual/widgets/_selection_list.py

@@ -204,22 +204,14 @@ class SelectionToggled(SelectionMessage[MessageSelectionType]):
 
         Can be handled using `on_selection_list_selection_toggled` in a subclass of
         [`SelectionList`][textual.widgets.SelectionList] or in a parent node in the DOM.
-
-        Note:
-            This message is only sent if the selection is toggled by user
-            interaction. See
-            [`SelectedChanged`][textual.widgets.SelectionList.SelectedChanged]
-            for a message sent when any change (selected or deselected,
-            either by user interaction or by API calls) is made to the
-            selected values.
         """
 
     @dataclass
     class SelectedChanged(Generic[MessageSelectionType], Message):
         """Message sent when the collection of selected values changes.
 
-        This message is sent when any change to the collection of selected
-        values takes place; either by user interaction or by API calls.
+        Can be handled using `on_selection_list_selected_changed` in a subclass of
+        [`SelectionList`][textual.widgets.SelectionList] or in a parent node in the DOM.
         """
 
         selection_list: SelectionList[MessageSelectionType]