Have you used Comments with Google Docs or Google Sheets?

Simply click in front of or highlight the section of the document you want to comment about and right click.  On the dropdown menu you will see Comment listed.  Click the comment listing and type your comment.  Now everyone you share the document with will see your comment.

Remove a comment by clicking on the comment.  Click the "3 dots" on the comment and click Delete.

You can also assign a comment as a task or alert a particular user to the comment.  When creating your comment start with a "+" sign followed by the user name of the person you want to alert or assign to.  Like this "+daniel.scissons@msa.state.mn.us" and type the comment.  The user or users you add get a personal alert to the comment.