If I read the automatic documentation for action=parse on en.wp, I have no way of telling that the parameters "mobileformat", "noimages" and "mainpage" come from the extension MobileFrontend and that that portion of the API is unstable as indicated here: https://www.mediawiki.org/wiki/Extension:MobileFrontend#API .
The API itself has no way to know this. MobileFrontend can do so in various ways, including
- Mentioning this in each parameter's existing documentation message.
- Using ApiBase::PARAM_HELP_MSG_INFO or ApiBase::PARAM_HELP_MSG_APPEND in the parameters' definitions to append a semi-standard message to each parameter's documentation.