Store extra, optional information about any entity
Suggestions

My first suggestion is that the business database is not the right place to store comments on what it contains.

Keep the comments in your issue tracker instead. A good issue tracker provides all the features to meet your requirements.

If you still want to do this in the database, research the history table pattern. See this Stack Overflow question for design ideas.

Essentially, you'd have an extra table for every table that you want to store comments about. Each table would refer to the business data by primary key, and would store the name, date, revision of the comment as well.

I've stated my assumptions below. Please correct me so we can help you better.

Assumptions

You have an issue tracker. It's regularly backed up and you can restore from backup.

You want to store instructive or descriptive text for some of the business data.

You would distinguish multiple comments by date of entry and by order of entry.

Most of the data would have no comments.

You want to be able to search the comments. Comment search queries run seldom compared to business queries.

You want to keep comments on deleted data.

You don't need to programatically cross-reference the business data and the comments or analyze them together. You just need to view and edit complete comments.

The comments do actually have value to the business (You are part of the business, right?).

Greater comment visibility would benefit the business.

Your business would react badly if the comments were lost.