[spec] Clarify purpose of alt-text

[Devin Prater]

This is my main issue: screen readers *do not* skip the block. This will
need to be dealt with by the client, not the screen reader.

On 2/28/21 9:06 AM, Adnan Maolood wrote:
> This would mean that should a screen reader encounter a preformatted text
> block with a non-empty alt text, the alt text would be read aloud and the
> preformatted block would be skipped. After all, that's what "alternative
> text" is supposed to mean.