Improve documentation for Alert and InlineAlert concerning semantics and ARIA live

[danielck]

The Problem

Current documentation is not clear on the differences of Alert and InlineAlert, especially regarding the semantics of the rendered HTML element and the intended use.

Proposed Solution

1. Make it clear that Alert elements will be announced automatically by screen readers and use the role=alert role by default.
2. Clarify that InlineAlerts do not use role=alert but are announced by screen readers by default.
3. Clarify usage of ariaLiveMode=off for disabling ARIA live functionality when used on a non-dynamic element.

Alternative Solutions

Consider whether ariaLiveMode=off should actually remove the aria-live attribute entirely from the rendered HTML, because that is the default anyway. The intended use of aria-live="off" in the ARIA spec is to stop certain elements (e.g. close buttons) within a larger ARIA live area from being announced by screen readers.

Additional Info

https://developer.mozilla.org/en-US/docs/Web/Accessibility/ARIA/ARIA_Live_Regions