Jonathan Bardo

Note: This plugin only works with WordPress 4.2 or higher
This plugin will enable the site administrator to easily create an admin message present on all pages. The admin has the option to let the user dismiss the message for the duration of his session.

Admin User Message

This does just what I want. Thanks for making it. :)
It's definitely better than https://wordpress.org/plugins/admin-screen-messages
You might want to also tag it as:
https://wordpress.org/plugins/tags/admin-message



Perfect

Add message to users of your site. Choose wheter they can dismiss it or not.

Jenst

Add a short note about the latest changes in the current post or page.
WP Update message adds a metabox in admin post or page section. If you updated a post or page with important information you can write a note about it here.
There are a few optional settings, but the plugin works fine even without changing anything.
<h4>Settings</h4>
You can choose a style theme and more.
<ul>
<li>Published date format (if published date should be included)</li>
<li>Updated date format (if updated date should be included)</li>
<li>Position</li>
<li>HTML</li>
<li>New row replacement</li>
<li>Style theme</li>
</ul>

WP Update Message

Add a short note about the latest changes in the current post or page.

Admin User Message vs WP Update Message