`may` and `should` now represent explicitly recommended or allowed
behavior.

`additional` contains other files whose applicability isn't made
explicitly clear, or which requires support for additional vocabularies.

`alternatives` contains "mutually contradicting" tests depending on
which choice was made by an implementation amongst a number of possible
options.

Each test in our previous optional/ directory is then sorted into one of
these directories.

It's definitions where there's language in the spec, dependencies
just appears in the metaschemas still.

…ADME doc language.