Ability to put comments on the text block

There are many times that some part of the text wants more attention while reading or even writing. Adding some markers could help to ease this situation.

Current approach

Using callout blocks or some distinguished block, that takes up a lot of space/structure and could even interfere with the content. Also it it tough to comment about some text selection. Which is also not neat when making notes.

Required approach

An ability to add comments to some text selection, and hover popup to view those comments.