New object attribute description-from

[aleventhal]

On the web, "description-from" has one one the following values:

• "aria-description"
• "aria-describedby"
• "ruby-annotation"
• "summary"
• "table-caption"
• "tooltip" (either via @title or aria-describedby + role="tooltip")
• "button-label" (when the name came from other markup such as aria-label)