Comment Syntax plugin
This plugin allows editors to add comments to the text of pages that will not be rendered on the page as displayed in the browser. They are useful for putting instructions in new page templates and the like. Individual lines in lists can be “commented out” without disrupting the list.
Several comment styles are supported, though the control-macro–style ones don't seem to work.
In fact, there are comments on this very page. They look like the ones reproduced in the code block below, but there is another set that you cannot see.
/* This is a C-style comment. They can be multiline and nested. */
/* Eh, I'll comment out the whole block.
/* This was the original comment describing the block */
Block
Block
*/
<!-- This is a HTML-style comment. They can be multiline and show up in the page source. -->