Add help text for rebalancer.

Our online help doesn't mention the effect of the metric
option.  This patch adds this information.  It also
adds the format for the refresh option.

Ticket: http://rt.dcache.org/Ticket/Display.html?id=8119
Requires-notes: no
Requires-book: yes
Target: master
Request: 2.7
Request: 2.6
Request: 2.2
Merge pull request #324 from paulmillar/fix/2.7/rebalancer-help

Add help text for rebalancer.