Many people (still) contribute PRs with undocumented members, or
undocumented classes, or undocumented defaults, because there is no tool
support to check they're not forgetting.

We used to have `@default` checking on props types for L2s, but not for
interfaces that are used INSIDE L2 props structs, or that are used as
props types for non-constructs.

Add two new checks:

* Requirement to document top-level types and public members.
* Requirement to supply `@default` for optional properties on structs.

Which supersede the previous one.

Obviously, we have many currently undocumented APIs which are added
to the ignore list.

Fixes #2286.

… time