docs: Add two missing entries in vm sysctl index

Both seem overlooked while adding the section in the main content.

Signed-off-by: Fam Zheng <famzheng@amazon.com>
Link: https://lore.kernel.org/r/20201022065403.3936070-1-fam@euphon.net


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