I think Properties are always single line, you have to use a free text item to have multiline. Is that an insurmountable restriction for you? Can you give an example where you would need it?
A trick I sometimes use is to write a few lines of text in a text editor, then copy it and paste it into the entry box of some other program.
This trick does not work if you edit a value field in the Symbol Properties window, ( When pasting a carriage return, it goes to the next field), but it does appear to work if you edit a single field directly on the schematic.
Note that this is is quite unusual to do and bugs may be lurking about. I have no idea what happens if you for example try to add this information to the BOM.