Modify comments at top of Custom CSS

[sterndata]

Users seen to put custom CSS into the first blank line of the custom CSS panel, nestling inside the comments, leading to a lot of "doesn't work" responses on the forum.

Current, it's

/*
You can add your own CSS here.

Click the help icon above to learn more.
*/

/* pointless addtional line */

I suggest a change like

/*************************************
You can add your own CSS with this tool.

Click the help icon above to learn more.

Add CSS below the following line 
**************************************/

[SergeyBiryukov]

Duplicate of #39892.