doc, mm: clarify /proc/<pid>/oom_score value range

doc, mm: clarify /proc/<pid>/oom_score value range

The exported value includes oom_score_adj so the range is no [0, 1000] as
described in the previous section but rather [0, 2000].  Mention that fact
explicitly.

Signed-off-by: Michal Hocko <mhocko@suse.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: David Rientjes <rientjes@google.com>
Cc: Yafang Shao <laoar.shao@gmail.com>
Link: http://lkml.kernel.org/r/20200709062603.18480-2-mhocko@kernel.org
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>