Suggest exposing <rt> as a description on the <ruby> base text

[aleventhal]

For <ruby>, most browser - screen reader combinations read redundant text, because the <rt> contents are exposed as well as the base text.

This situation can be handled by move <rt> contents to accessible descriptions applied to the base <ruby> text in the accessible tree. The screen reader or user will then hear only the base text by default, but can still access the <rt> text.

In addition, "description-from" is set to "ruby-annotation" (see #351 for an explanation of description-from, which is implemented in some browsers but not yet part of HTML-AAM). This allows the screen reader to disambiguate from other types of descriptions and apply different verbosity rules.

Note that this is how Chrome already exposes ruby and rt.