Edits and refinements to 1.3.1 release notes.

Docker-DCO-1.1-Signed-off-by: Fred Lifton <fred.lifton@docker.com> (github: fredlf)
Added missing item from Changelog

Docker-DCO-1.1-Signed-off-by: Fred Lifton <fred.lifton@docker.com> (github: fredlf)
Merge pull request #8892 from fredlf/relnotes-edit

Edits and refinements to 1.3.1 release notes.