Note that the original topic of the thread was for version 4 of kicad. Version 5 looks quite different.
If you run nightly which will become 5.1.x then you can checkout my tutorial for creating symbols (I handle adding descriptions in it.) Tutorial: How to make a symbol (KiCad v5.1.x)
If you run current stable then use the symbol properties dialog reachable via the top menu (symbol->properties, switch over to the description tab)
Or a button in the top toolbar (looks like a gear, next to the button of an upercase T)
