I wrote a "How to" post about this a couple years ago:
Select the Field item in Sitecore that you want to add help text to and open the "Help" section (you may need to toggle visibility on Standard Fields).
The Help Link field will be used to provide a clickable link on the field label. I find it really helpful to link fields directly to documentation explaining what the field is used for. This is particularly useful for fields that don't change too often, or are optional.
The Long Description field is used to provide a tooltip for the field label. Don't use HTML in this field, it will be escaped in the tooltip.
The Short Description field is used to provide the helper text that follows the field. Don't use HTML in this field, it will be escaped when shown in the helper text. Worse yet, it won't be escaped where it's shown at the top of the Field item.
In general, avoid
& characters in the help text fields because Sitecore doesn't handle the HTML encoding consistently.
Also, don't be fooled by the "Help Texts" dialog if you use the Help button in the ribbon. It's mislabeled. Long Description is the tooltip, Short description is what follows the field.