Can I write comments in UML Class diagram?

Use --.

...if you are looking to comment the UML source, rather than render a comment in a diagram. As mentioned by WileCau above the reference for this is here: http://www.omg.org/spec/OCL/2.2 and is called an Object Constraint Language comment."

-- This section is about [etc.]
[My Thing] -> (Result)

This doesn't work in all UML renderers though - When I tried in PlantUML for example, it considered this a syntax error.

If you looking to add a comment in the UML source, depending on the renderer you can use -- or ' for a single line comment.

For PlantUML comments:

@startuml
' This is a comment on a single line
Bob->Alice : hello
/' You quote alors use slash-and-quote
to split your comments on several
lines '/
@enduml