I figured it out! For others looking for the answer, see below:
In order to allow a tooltip to accept HTML, do this -
- Go to Custom Attributes and create a new Tooltip
Field > Custom Attributes
Key = tooltip
Value = Tooltip text. <add your HTML>
- Then add a new custom attribute below the first tooltip attribute like this
Key = allow_html
Value = true
By adding the second attribute for "allow_html = true" then Tooltips will now accept HTML.