There is a Comment feature available under Insert.
You cannot (yet) put a box with text in a googledoc unless it is made as a drawing.
Note that
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From the Insert tab, go to the Text group and click the Text Box Button. Select a Built-In text box or click "Draw Text Box".
Go to Menu Bar click “Insert” and then Go to “Drawing” and click it. You will See + New and From Drive. Click on + New. It will open a new window called Drawing. As shown in below image.
Click on Text Box icon which appears in the toolbar. Go to the Drawing Panel. Click and Drag your mouse to create a Text box Space and resize it according to your use.