Chromium should have a place where documentation guidelines live and can be referred to.

I'm in the process of drafting some guidelines and will add them to src/docs/ soon.

It's worth noting that 
https://chromium.googlesource.com/chromium/src/+/master/docs/documentation_best_practices.md
exists already and has some good advice in it though it's also a bit google-internally.

I will see about merging them after landing the first thing.