Reorganize "implicit value for role" in the spec to clarify differences from IDL defaults

[cookiecrook]

@annevk and I were discussing the IDL fix-ups and realized the spec will need to clarify distinctions between:

• the IDL default (including missing/invalid states), which cannot change per role, and
• the accessibility implementation defaults, which can change per role, but won't reflect in the IDL, as they are an implementation detail of the browser accessibility internals. These are the ones listed in the ARIA spec as "Implicit Value for Role."

Given aria-orientation as an example, the attribute characteristics table does not make any mention of these Implicit Values. There is an informative note that mentions the differences for slider (horizontal) and scrollbar (vertical), but there is no single listing anywhere in the spec. There's also an orientation default on listbox (and many others), but you have to either know that or perform an awkward substring search for "Default for aria-orientation is"

We will need to add the default/missing/invalid IDL values to the attribute's characteristics table, so having some seemingly conflicting accessibility values in the role's characteristics table elsewhere may lead to confusion among both implementors and authors.

I propose all the normative accessibility " implicit defaults" in the spec be moved to the attribute's characteristics table. So aria-orientation will list its values, including the IDL default/missing/invalid values, as well as the accessibility implementation defaults, which will not be reflected in the IDL. Some explanatory text of this distinction will need to be added in one or more places...

The roles with non-default values should also have informative notes cross-referencing these normative defaults defined in the attribute. Ideally we could do this in a ReSpec function to reduce duplication.