docs: mm: fix vm overcommit documentation for OVERCOMMIT_GUESS

Commit 8c7829b0 "mm: fix false-positive OVERCOMMIT_GUESS failures"
changed the behavior of the default OVERCOMMIT_GUESS setting.
Reflect the change also in the Documentation, namely files:
    Documentation/admin-guide/sysctl/vm.rst
    Documentation/mm/overcommit-accounting.rst

Reported-by: Jozef Bacik <jobacik@redhat.com>
Signed-off-by: Vratislav Bendel <vbendel@redhat.com>
Acked-by: Mike Rapoport <rppt@linux.ibm.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20220829124638.63748-1-vbendel@redhat.com