feat(material/checkbox): add new aria properties to MatCheckbox

Added three new aria properties to MatCheckbox:
`aria-expanded`: Indicates whether the checkbox controls the visibility
of another element. This should be a boolean value (true or false).
`aria-controls`: Specifies the ID of the element that the checkbox controls.
`aria-owns`: Specifies the ID of the element that the checkbox visually owns.
These attributes will be added to the generated checkbox element if they
are specified and won't be present in the HTML if not provided.
Also added a small paragraph at the end of the checkbox.md file.

Fixes angular#28761

src/material/checkbox/checkbox.html

@@ -10,9 +10,9 @@
            [attr.aria-labelledby]="ariaLabelledby"
            [attr.aria-describedby]="ariaDescribedby"
            [attr.aria-checked]="indeterminate ? 'mixed' : null"
+           [attr.aria-controls]="ariaControls"
            [attr.aria-disabled]="disabled && disabledInteractive ? true : null"
            [attr.aria-expanded]="ariaExpanded"
-           [attr.aria-controls]="ariaControls"
            [attr.aria-owns]="ariaOwns"
            [attr.name]="name"
            [attr.value]="value"