There may be plugins that supply some kind of commenting functionality, similar to how Google Docs allows you to annotate parts of a document.
Within WordPress directly, you could use the “Text Editor” view and HTML comments. The downside here is that the HTML comments will be visible if someone goes to the page and uses their browser’s “View Source” option which shows the unrendered HTML.
An example of post content with comments would be
<!-- No one can see this when the page is rendered -->
Some regular content in the WordPress editor.
An alternate method:
Define a custom shortcode that doesn’t render.
You could create a custom shortcode that doesn’t output anything. So, in your editor you’d have:
[comment]This content will not render to the post[/comment]
And then in your functions.php or somewhere else, add the following code:
add_shortcode( 'comment', '__return_empty_string' );
Then, whenever the shortcode is processed in the content, it will just return an empty string.