docs(dev-infra): improve commit message guidelines (#1417)

This change provides more clarity as to the type and scope requirements,
and adds in additional information with regard to hyperlinks,
appropriate trailers to
use in the footer, and clarifies the difference between our requirements
and the conventional commit spec v1.0.0.

Change-Id: I7e3d4646c09728acf27c4efce24655896b8513a7