The issue is more that the Chromium OS Sheriff FAQ (which the letter points to) isn't a great introduction to being a sheriff.

The first mention of buildbot waterfalls are in the 3rd section.  In the 5th section there are explanations of the various builders, but not in the context of the waterfall.

A separate document explains the Chromium (browser) waterfall, but it's not directly linked here, and it's less useful because the builders are all different.

New sheriffs will benefit from a visual explanation of the cros tree and simple examples of what to do. 

+ Ketaki who I believe is looking into this 

Yes. We have a small taskforce of Mqg@ and me on this :). We will work on make this better.