Just remember that the double slash only allows for one line of text. If you add a line without the double slashes or the line you have somehow jumps to the next line by mistake... error!
In case you're wondering, you can set apart an entire paragraph as a comment. Just start the text with this: /* and end it with this: */
. Keep the star next to the text. Then you can write as many lines as you want. The browser will see it all as a comment rather than something to be played with.