(this continues #11442) There is no more need to install a new compiler. This simplifies instructions a lot.
From discussion with @fanquake on IRC I first wanted to add a new section for 6.2, but that made the document a complex mess. I think it's good enough (and more maintainable too) to only support the most recent release.
Includes #11976.
I moved the "resource limits" section to the end as I didn't seem to need it with clang, but this may vary based on source changes and the phase of the moon so it's good to keep it as optional extra information.