Explain Brad's algorithm for generating commit headlines.

Fixes #15700

Change-Id: Ic602f17629b3dd7675e2bb1ed119062c03353ee9
Reviewed-on: https://go-review.googlesource.com/23355Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>