IS DISTINCT FROM not documented

On one of the other lists a suggestion was made to use the IS DISTINCT FROM
operator. I didn't remember seeing anything about this before so I tried
to look it up in the 7.4.2 documentation. After not finding anything, I tried
grepping the documentation source and was only able to find a reference
to it in the release notes for 7.3. It may be that because of line breaks
or mark up that I missed it. But even if it is there, it doesn't seem to
be in an obvious place.