How can I add an info text to the custom extra fields? Just for explanatory and other info.

Seems I'm missing a way to add information text between the custom fields. Being able to add html codes with the info text would be really handy too!

So it’s not a field box for users to fill in. Just extra text for explaining purposes, above/below the extra fields.

You could try using the Header extra field type to do this, to adjust the H4 tag, do a template override and change to whatever like <p> or <em> and style it with css.