docs(focus): Add docs for regions #8082 (#8128)

* docs(focus): Add docs for regions #8082

* docs(a11y): add more desc for focus trap

Author: amcdnl

src/cdk/a11y/a11y.md

@@ -68,6 +68,22 @@ This directive is declared in `A11yModule`.
 This directive will not prevent focus from moving out of the trapped region due to mouse
 interaction.
 
+#### Regions
+Regions can be declared explicitly with an initial focus element by using 
+the `cdkFocusRegionStart`, `cdkFocusRegionEnd` and `cdkFocusInitial` DOM attributes. 
+`cdkFocusInitial` specifies the element that will receive focus upon initialization of the region. 
+`cdkFocusRegionStart` and `cdkFocusRegionEnd` define the region within which focus will be 
+trapped. When using the tab key, focus will move through this region and wrap around on either end.
+
+For example:
+
+```html
+<a mat-list-item routerLink cdkFocusRegionStart>Focus region start</a>
+<a mat-list-item routerLink>Link</a>
+<a mat-list-item routerLink cdkFocusInitial>Initially focused</a>
+<a mat-list-item routerLink cdkFocusRegionEnd>Focus region end</a>
+```
+
 
 ### InteractivityChecker
 `InteractivityChecker` is used to check the interactivity of an element, capturing disabled,