Update definition for "status message"

[patrickhlauke]

The current definition for "status message" https://www.w3.org/TR/WCAG22/#dfn-status-messages manages to somehow miss out the fact that a status message refers to an actual discrete "message". The current definition makes it sound like any change of content may be interpreted as an "intrinsic" status message (see discussion #4672).

This PR changes the (normative) definition to make it clear that the WG always intended this to refer to explicit "status messages", and not somehow mandate the need for any dynamic content update (such as dynamically updating a list of results) must trigger some sort of SR announcement.

Closes #4676

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[patrickhlauke]

Slotting the new definition in place of the term into the normative wording for the SC, results in:

In content implemented using markup languages, discrete pieces of content that are changed/updated dynamically and provide information to the user on the success or results of an action, on the waiting state of an application, on the progress of a process, or on the existence of errors can be programmatically determined through role or properties such that they can be presented to the user by assistive technologies without receiving focus.

(note that the definition is for "status message", singular, while the SC uses "status messages", plural, so I tweaked the replacement accordingly to make it plural)

compare this to the far more open-ended/vague current definition

"In content implemented using markup languages, changes in content that are not changes of context, and that provide information to the user on the success or results of an action, on the waiting state of an application, on the progress of a process, or on the existence of errors can be programmatically determined through role or properties such that they can be presented to the user by assistive technologies without receiving focus."