Or add an expression for which you disable all Display Options, except "Text as Pop-up". You are now free to produce any text as expression result and it will be shown in the tool-tip. This can be a complex assembly of text, variables and expression results (include chr(10) to get different lines)
Don't forget to disable "Pop-up labels" on the Properties->Presentation tab. Otherwise, the default pop-up with label text will still be displayed.