You should always provide a descriptive name for the variable that will provide for its meaning. You can also, if needed, add a note (Tools menu) on the page the variable is assigned a value. Notes can only be seen in edit mode.
That’s fine if there is only handful of custom variables but in titles with numerous custom variables a descriptive name won’t necessarily be unambiguous e.g. is the custom variable “usersname” the user’s full name, just their first name, in the format last name comma first name…
Lectora already provides a dialog box to manage variables. Surely having a text box to add a description for a variable is less error prone and more sensible than any other way?
Should be possible, there’s a Description field for Reserved variables as well.
You can of course use Darrels suggestion or just write all your infos to a normal textfield and keep it outside the stage. Then you can use all the normal formatting and inheritance options, so it should be more easily maintainable than a description field in the variable manager.