man/vzctl.8: explain 'q' key for devices

A 'q' key which turns on quota management inside
a container was not documented (except for
the source code and a few posts on forum).

http://bugzilla.openvz.org/1320

[kir@openvz.org: slightly modified the text]

Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
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