docs: More precisely describe memory-backend-*::id's user

'id' of memory-backend-{file,ram} is not only for '-numa''s reference, but
also other parameters like '-device nvdimm'.
More clearly call out this to avoid misinterpretation.

Signed-off-by: Robert Hoo <robert.hu@linux.intel.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <1619080922-83527-1-git-send-email-robert.hu@linux.intel.com>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>