You can insert a text box to a Google Docs document and here's how you can do that on PC:
That's all. Now, you can see the text box in the place you've preferred in the document.
You can't do this task on Google Docs for Android.
You can edit the text box at any time you want and here's how you can do that - Select the text box you want to edit and then click on "Edit" link. Now, you can see the "Drawing" popup where you can make the changes you want.
I hope that this post is useful to you. If it is, share this post with your friends.
How to Insert Text Box in Google Docs