docs/vm: clarify overcommit amount sysctl behavior

docs/vm: clarify overcommit amount sysctl behavior

Documentation/vm/overcommit-accounting.rst says that the overcommit
amount can be set via vm.overcommit_ratio and vm.overcommit_kbytes.

Add a clarification that those only take effect in overcommit handling
mode 2 ("Don't overcommit"), i.e. they do not act as an "additional"
limit that is always enforced.

Signed-off-by: Anssi Hannula <anssi.hannula@iki.fi>
Reviewed-by: David Hildenbrand <david@redhat.com>
Link: https://lore.kernel.org/r/20211211194159.3137362-1-anssi.hannula@iki.fi
Signed-off-by: Jonathan Corbet <corbet@lwn.net>