xl: xl.conf(5): correct advice re autoballooning vs. dom0_mem.

The advice was backwards, you should really disable autoballoon if you
use dom0_mem. Also add a reference to the command-line docs.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Committed-by: Ian Campbell <ian.campbell@citrix.com>

docs/man/xl.conf.pod.5

@@ -49,9 +49,9 @@ The semantics of each C<KEY> defines which form of C<VALUE> is required.
 
 If disabled then C<xl> will not attempt to reduce the amount of memory
 assigned to domain 0 in order to create free memory when starting a
-new domain. You should set this if you use the C<dom0_mem> hypervisor
-command line to reduce the amount of memory given to domain 0 by
-default.
+new domain. You are strongly recommended to set this to C<0>
+(C<False>) if you use the C<dom0_mem> hypervisor command line to
+reduce the amount of memory given to domain 0 by default.
 
 Default: C<1>
 

docs/misc/xen-command-line.markdown

@@ -440,6 +440,11 @@ reservation is unlimited.
 For example, to set dom0's initial memory allocation to 512MB but
 allow it to balloon up as far as 1GB use `dom0_mem=512M,max:1G`
 
+If you use this option then it is highly recommended that you disable
+any dom0 autoballooning feature present in your toolstack. See the
+_xl.conf(5)_ man page or [Xen Best
+Practices](http://wiki.xen.org/wiki/Xen_Best_Practices#Xen_dom0_dedicated_memory_and_preventing_dom0_memory_ballooning).
+
 ### dom0\_shadow
 > `= <boolean>`