SUPPORT.md: write down restriction of 32-bit tool stacks

Let's try to avoid giving the impression that 32-bit tool stacks are as
capable as 64-bit ones.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Julien Grall <jgrall@amazon.com>

SUPPORT.md

@@ -168,6 +168,12 @@ ARM only has one guest type at the moment
 
 ## Toolstack
 
+While 32-bit builds of the tool stack are generally supported, restrictions
+apply in particular when running on top of a 64-bit hypervisor.  For example,
+very large guests aren't expected to be manageable in this case.  This includes
+guests giving the appearance of being large, by altering their own memory
+layouts.
+
 ### xl
 
     Status: Supported