types: Improve the version specific SSOT docs

The module docs in each version specific module (e.g. `types::v17`) are
supposed to be the single source of truth for devs trying to work out
which methods are supported and how.

Make an effort to better present the information.

Note that a bunch docs for methods marked as TODO may be incorrect re
version vs version + model. This doesn't matter right now since they
will be checked when implemented and also TODO implies no guarantees (in
my mind anyway).

Note also issue #144. `verify` does not check the Returns column against
the `Method` constructor type.

Close: #109

Author: tcharding