This is not in the manual, because it was added in a later release. You can find it in PF3/Help/version_history.pdf in the changes for version 3.5.0:
CHANGED the altitude change request to now include a request to remain
at your current altitude by using either the 'H' or 'L' key together with the
ZERO key. Since this could be a request due to problems in attaining your
last cleared altitude, ATC will always approve the request (there is a small
percentage of being told 'unable at this time' for other requests). It is
recommended you are levelled off when making this request.