docs: clean up sysctl/kernel: titles, version

This cleans up a few titles with extra colons, and removes the
reference to kernel 2.2. The docs don't yet cover *all* of 5.10 or
5.11, but I think they're close enough. Most entries are documented,
and have been checked against current kernels.

Signed-off-by: Stephen Kitt <steve@sk2.org>
Link: https://lore.kernel.org/r/20201208074922.30359-1-steve@sk2.org


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