ThemeNegotiator should document that the first ThemeNegotiatorInterface service which returns a theme will win

Problem/Motivation

The docs for ThemeNegotiator don't explain how the different negotiator services interact, and while ThemeNegotiatorInterface has more docs, it says:

> make sure to set the priority on
> * the service to a high number so that it is not accidentally overridden

Which is *sort of* true, but not actually true, in that it doesn't get 'overridden', but rather, the first negotiator to return a theme wins and subsequent ones aren't even checked.

Steps to reproduce

Proposed resolution

- ThemeNegotiator should have a @see to ThemeNegotiatorInterface where the main docs are
- ThemeNegotiatorInterface should explain that implementing services are checked in priority order and the first one to say it applies AND return a theme, wins.

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet