docs(material/list): update a11y documention on radio indicators

Update the accessibility section on radio indicators for
single-selection. Add instructions to always communicate selection with
icon indicators. Fulfill documentation needs as follow-up for angular#25933 and
 angular#26283.

Author: zarend

src/material/list/list.md

@@ -213,6 +213,10 @@ these options, including buttons and anchors.
 Always provide an accessible label for the `<mat-selection-list>` element via `aria-label` or
 `aria-labelledby` that describes the selection being made.
 
+`MatSelectionList` visually communicates selected state with both change of color and icon
+indicators. By default, single-selection displays radio icon indicators. Always shows icon
+indicators for selection.
+
 #### Custom scenarios
 
 By default, the list assumes that it will be used in a purely decorative fashion and thus it sets no