Show changelog for core upgrade

[petervanderdoes]

When clicking on the admin notice that there is an update available, it would be interesting to see a short changelog with a link to the complete changelog of the upgrade besides the two download buttons

Example with the 2.8.5.
Show on the page:
The headline changes in this release are:

• A fix for the Trackback Denial-of-Service attack that is currently being seen.
• Removal of areas within the code where php code in variables was evaluated.
• Switched the file upload functionality to be whitelisted for all users including Admins.
• Retiring of the two importers of Tag data from old plugins.

More info : ​http://wordpress.org/development/2009/10/wordpress-2-8-5-hardening-release/

[edwardw]

Outdated, stale. Relevant progress: #18742

[nacin]

Re-opening this as #18742 applies to post-update, while this is pre-update.

[chriscct7]

This is a duplicate of #18742 and the linked ones on that ticket. Essentially this was implemented after the update as opposed to before it.