Update a11y documentation

[oliverfoster]

Subject of the issue

Adapt should be able to read "$5bn" as "five billion dollars". This is a slightly different use-case to abbreviations which should be pronounced correctly whereas here an entirely alternative text should be read.

Can be solved with a helper.

[moloko]

Could we have an option to make it read like this?

[moloko]

Reminder for one of us to update the a11y documentation to include something about this in the a11y wiki when released.

Have a feeling that the zero-width space stuff might also need writing up...?

[oliverfoster]

Also the helpers need updating in the WIKI

[kirsty-hames]

Reminder for one of us to update the a11y documentation to include something about this in the a11y wiki when released.

Have a feeling that the zero-width space stuff might also need writing up...?

a11y_alt_text documented here on the wiki but zero-width needs adding.

[oliverfoster]

duplicate of #3379