Just came across this http://www.theregister.co.uk/2016/08/08/linux_48_rc1_lands_with_surface_3_support_promised/:

Torvalds has also pointed out that while progress on v 4.8 is “fairly normal … the patch itself looks somewhat unusual: over 20% of the patch is documentation updates, due to conversion of the drm and media documentation from docbook to the Sphinx doc format. There are other doc updates, but that's the big bulk of it.”

DocBook is an open-source implementation of SGML, which is a pretty heavyweight, official-international-standard, 1980s, big-company way of writing documentation. Sphinx, on the other hand, is commonly used for Python documentation, including on python.org itself https://docs.python.org/3/index.html.

If you were putting together the documentation set for a passenger liner or a chemical factory, you’ll probably use SGML (in fact, there is very likely a legal requirement to do so). But for the rest of us, more lightweight markup systems like Sphinx/ReST and Markdown are much more convenient.