Clean up `.tools.messagev` docs

[khaeru]

As part of #207, code in .tools.messagev and associated docs was migrated from message_data.
This is mainly because it is imported by .model.transport.migrate, but the latter is not tested, and neither is the former; so it is currently-unused utility code.

In reviewing #208, @glatterf42 noted (here and here):

• This :doc: reference [to import_from_msgV_sqlite] does not seem to build a link in the final docs page.
• Might be nice to render these quoted options as code.

This issue is to remind that these clean-ups, and probably others, can be made to the docs.