The Tool PaletteToolbox for UML diagram elements displays note and note link buttons for all UML diagrams. Use these elements to place annotation nodes and their links on the diagram. 

Notes can be free floating or you can draw a note link to some other element to show that a note pertains specifically to it. 

You can attach a note link to another link. 

The text of notes linked to class diagram elements does not appear in the source code.