docs: Mention a common pitfall in ballooning

docs: Mention a common pitfall in ballooning

Several users have reported that their available free memory in the
guest when they used maxmem >> memory was much smaller than when
maxmem == memory.  This is the unavoidable consequence of how
ballooning works, but it's not something users expect.

Warn them of this effect in the place we tell them how to make it
happen, so they aren't surprised.

Signed-off-by: George Dunlap <george.dunlap@eu.citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>