According to the conventions about writing system messages, all messages must have clear and relevant documentation in qqq.json. It is helpful to translators, and it is also useful to other developers who need to understand how messages are used.

At the moment, many of the REST API messages have practically no qqq documentation. Many of them just repeat the English text of the message, which is completely useless.

REST API messages should probably be documented similarly to the action API messages. For the action API help messages, translatewiki has several templates that describe what kind of message it is: action, error, example, parameter, etc. You can see a list of those templates at https://translatewiki.net/wiki/Category:MediaWiki_API_Message_Documentation_Templates . Because these messages are somewhat repetitive, the templates are often enough, but if any more information is needed about a particular message, it can be just written as additional text in qqq.

The REST API should probably use a similar system. I know very little about the REST API, so I cannot offer it myself. I'll be happy to work with the REST API developers to come up with a grouping pattern and with writing the necessary templates.