meta-refresh rules, add explanation of meta element [bisz58] [bc659a]

[WilcoFiers]

[Feedback from AGWG])(w3c/wcag#3549 (reply in thread))

Since meta refresh is a fairly dated technique, it would be useful to add a brief explanation of what it does and how it's used to the background of these two rules:

• Meta element has no refresh delay
• Meta element has no refresh delay (no exception)