Documentation: scheduler: fix outdated information on sched groups

The documentation claims that two sched groups must not overlap. This is
no longer true, as overlapping sched groups are used on NUMA systems.
This change has been introduced by commit e3589f6c and was
documented by an in-code comment in commit 35a566e6.

Signed-off-by: Adrian Freund <adrian@freund.io>
Link: https://lore.kernel.org/r/20200407130525.76663-1-adrian@freund.io


Signed-off-by: Jonathan Corbet <corbet@lwn.net>