spacemanager: Fix help output of reserve space

@Argument help is currently not rendered as part of the help
output. This patch moves the mentioning of the optional suffix
to the main description.

Target: trunk
Require-notes: no
Require-book: no
Acked-by: Paul Millar <paul.millar@desy.de>
Patch: http://rb.dcache.org/r/6454/