The problem, as I understand it, is that these are summaries of summaries. The actual commit messages are incredibly detailed, then they get rolled up by the lieutenants into a single merge with one line per change, then Linus merges them in and adds to the final summary 'networking updates'. If you want to know what actually changed, then look at the commit logs, e.g.:

https://github.com/torvalds/linux/compare/v4.9-rc8...v4.9

Linus provided a TL;DR: A bit over two thirds drivers (staging, GPU and networking are the bulk of it, but it's all over), with the rest looking fairly normal too: arch updates, documentation, generic networking, filesystems..