Most parameters can take multiple lines of text, this can be done by pressing CTRL return to start a new line or by copy and pasting multi-line text into a parameter value.
The problem is that Revit won't display multi-line parameter values properly in the Property Palette or in a schedule view - this makes it very difficult review or edit.
For examples:
Multi-line values as viewed in the Properties Palette:

Multi-line values as viewed in a Schedule view:

Multiple lines are only displayed properly when displayed on a sheet:

When working with existing buildings its useful to write out the works to various objects in a schedule, its a nightmare to edit the schedule when you're unable to read the text in the view your entering it.
This is a basic function of CAD software and should be fixed...