In node bodies, you can use <!--break--> to decide the teaser chop off point. It is in the form of an HTML comment for historical reasons. Today however, there is no need for this tag to be a comment. It is removed from the node body and teaser when output, so it works in BBcode, textile or any other format too.
So, let's change it to the easier to use
<break>. HTML is the most used format around, and it does no harm in other formats anyway.
The attached patch maintains backwards compatibility with the old break tag, but the help texts only speak of