doc: clarify libnuma requirement for NUMA systems

doc: clarify libnuma requirement for NUMA systems

Since 18.05, libnuma is pretty much required on Linux when using
non-legacy mode, because without it, we cannot know where our
hugepages are located [1].

In legacy mode, libnuma is not required because we can still sort
pages by sockets, as we use pagemap lookup method to figure out
socket ID's for pages.

So, document libnuma as required for NUMA systems and non-legacy
mode.

[1] https://mails.dpdk.org/archives/dev/2018-December/120490.html

Fixes: 6b42f75632f0 ("eal: enable non-legacy memory mode")
Cc: stable@dpdk.org
Signed-off-by: Anatoly Burakov <anatoly.burakov@intel.com>
Acked-by: Kevin Traynor <ktraynor@redhat.com>