Update 2.12 release notes with fixes from stable-2.11
Update 2.12 release notes with fixes from stable-2.11

Version 2.12 included several fixes that were made on stable-2.11
and merged up to stable-2.12 before 2.11.6 was released. Other
fixes were made on stable-2.11 after 2.12 and were included in
2.11.6.

In this case we can't simply say that "2.12 includes fixes done
in 2.11.6" because it doesn't include all of them.

Update the release notes to explicitly mention the ones that got
included.

Change-Id: Ie996d10012e5060fb7b3f07d0c80c6cd2cfeeb4e

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