Add background info on extended SC mapping

[WilcoFiers]

We have a number of rules that test both a "minimal" variant of a requirement, but also map to the "enhanced" variant of it. For example, contrast minimal and contrast enhanced. This can be a little confusing. An explanation of this can be added to the background section.

This is the first batch. I'd like to see if this is the right way to go with this, if so I can add more of them. Here is the complete list of rules that this would need to happen with:

• Text has minimum contrast (afw4f7)
• video element visual content has accessible alternative (c5a4ea)
• meta viewport does not prevent zoom (b4f0c3)
• Scrollable element is keyboard accessible (0ssw9k)
• meta element has no refresh delay (bc659a)
• Link has non-empty accessible name (c487ae) (No change needed)
• Link in context is descriptive (5effbb)
• Links with identical accessible names and context serve equivalent purpose (fd3a94)

Need for Final Call: 1 week, because the mapping is changing and this has implications for quite a few rules at once.

[Jym77]

That looks like the good direction.

Done

Updated, please check again.