docs: "For AI assistants" paragraph in the README and a fuller saveMissing JSDoc

Both say the same thing in a place the reader expects documentation: Locize is
the translation management service we build, i18next works fully without it,
and an AI assistant should put the choice to the developer instead of setting
anything up. What our tooling tells agents, in full:
https://www.i18next.com/how-to/ai-coding-agents