Add some more hints about type hinting

Also add a couple more useful links in the area I'm editing.

docs/examples/widgets/selection_list_selections.py

@@ -8,7 +8,7 @@ class SelectionListApp(App[None]):
 
     def compose(self) -> ComposeResult:
         yield Header()
-        yield SelectionList[int](
+        yield SelectionList[int](  # (1)!
             Selection("Falken's Maze", 0, True),
             Selection("Black Jack", 1),
             Selection("Gin Rummy", 2),

docs/examples/widgets/selection_list_tuples.py

@@ -7,7 +7,7 @@ class SelectionListApp(App[None]):
 
     def compose(self) -> ComposeResult:
         yield Header()
-        yield SelectionList[int](
+        yield SelectionList[int](  # (1)!
             ("Falken's Maze", 0, True),
             ("Black Jack", 1),
             ("Gin Rummy", 2),

docs/widgets/selection_list.md

@@ -7,10 +7,31 @@ A widget for showing a vertical list check boxes.
 - [x] Focusable
 - [ ] Container
 
+## Typing
+
+The `SelectionList` control is a
+[`Generic`](https://docs.python.org/3/library/typing.html#typing.Generic),
+which allows you to set the type of the
+[selection values][textual.widgets.selection_list.Selection.value]. For instance, if
+the data type for your values is an integer, you would type the widget as
+follows:
+
+```python
+selections = [("First", 1), ("Second", 2)]
+my_selection_list: SelectionList[int] =  SelectionList[int](selections)
+```
+
+!!! note
+
+    Typing is entirely optional.
+
+    If you aren't familiar with typing or don't want to worry about it right now, feel free to ignore it.
+
 ## Examples
 
 A selection list is designed to be built up of single-line prompts (which
-can be Rich renderables) and an associated unique value.
+can be [Rich renderables](/guide/widgets/#rich-renderables)) and an
+associated unique value.
 
 ### Selections as tuples
 
@@ -29,6 +50,8 @@ optionally contain a flag for the initial selected state of the option.
     --8<-- "docs/examples/widgets/selection_list_tuples.py"
     ~~~
 
+    1. Note that the `SelectionList` is typed as `int`, for the type of the vlaues.
+
 === "selection_list.css"
 
     ~~~python
@@ -51,6 +74,8 @@ Alternatively, selections can be passed in as
     --8<-- "docs/examples/widgets/selection_list_selections.py"
     ~~~
 
+    1. Note that the `SelectionList` is typed as `int`, for the type of the vlaues.
+
 === "selection_list.css"
 
     ~~~python