We have two main documents for i18n:

1. GUIDELINES: for developers and translators to look at all the ICU syntax we support with examples, it also includes our best practices and how to write the message ID properly to give context to the translators about the string being translated.
2. README: Contains the technical details for developers to use i18n in their plugins, and also documents the kbn-i18n and kbn-i18n-react packages features.

With the i18n new system upgade comes new ICU syntax, deprecated functions, different flows for unit testing, different flows for psuedo localization, and a lot of changes to the packages we have. This is all needs to be documented.