docs: renice(1): Rework discussion of unprivileged users,

docs: renice(1): Rework discussion of unprivileged users,

The BUGS section describing the limitations on what an unprivileged
user may do to the nice value is outdated, given the kernel changes
that added RLIMIT_NICE in Linux 2.6.12. So, remove that text.

The revised details for modern Linux were partially covered in NOTES,
but there were also inaccuracies there (and the use of the word
"monotonically" was unneeded). In particular, the point is that
unprivileged users can only increase the nice value. There is no
restriction particular to the range 0..+19. So, for example, the
following scenario is possible:

1. Superuser sets the nice value of an unprivileged user's
   process to -20.
2. The unprivileged user can now renice that process to (say) -10.

Signed-off-by: Michael Kerrisk <mtk.man-pages@gmail.com>
Signed-off-by: Karel Zak <kzak@redhat.com>