Add a note to educate on keystone's two ports

As suggested by Dung K Hoang, readers find the use of two different
ports in keystone confusing. This patch adds a short explanatory note

Change-Id: I17bccf993f6dc12c1e2a1f9099fbddf53dd1b202
Closes-Bug: 1384970
(cherry picked from commit 046498d7bed159ae237505ace0ebe724107a6158)