Make the "Post Author" description context-aware

[jameskoster]

Blocks like Post Author are required to entertain multiple contexts:

1. While editing a template the block is generic and not associated with any single post
2. If inserted in a page, the block will display the author of that page
3. If inserted in a Query in that same page, the block will display the author of the post(s) returned by the Query

The current description "Add the author of this post." could use some elaboration.

It would be nice if the description could vary based on the scenarios above.

Scenario	Description
Editing a template	Displays the author of the post
Inserted at the root level of post "Hello World!"	Displays the author of this post
Inserted in a Query	Displays the author of post name

Other blocks that will likely need the same treatment:

• Post Content
• Post Date
• Post Excerpt
• Post Featured Image
• Post Tags
• Post Categories

[jameskoster]

I think this is covered by #27064