There isn't much documentation apart from the scripts that create a release for creating a MediaWiki release.
As a result, there were some failures that could have been avoided -- branching the extensions at the same time core was branched, for example.
To improve the process for any future releases, we need to document the process. To that end, I've started https://www.mediawiki.org/wiki/Tarball_release_process to begin to collect ideas and, ultimately, provide a recipe for making a release that anyone can follow.
Please help me improve future releases by adding to the page.