mem: document callbacks not being supported in some cases
mem: document callbacks not being supported in some cases

Mem event and validator callbacks may not be supported under all
circumstances (such as when running in legacy memory mode, or on
FreeBSD), and this case needs to be handled by any code that will
use these callbacks. Spell this out more clearly, because it's not
immediately obvious that this is an expected use case.

Signed-off-by: Anatoly Burakov <814e00d8f6e4e81b7ccc7e3e5a0bfec0078f0203@intel.com>

john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk