trivial: doc: document missing value 2 for randomize-va-space

The documentation for /proc/sys/kernel/* does not mention the possible
value 2 for randomize-va-space yet.  While being there, doing some
reformatting, fixing grammar problems and clarifying the correlations
between randomize-va-space, kernel parameter "norandmaps" and the
CONFIG_COMPAT_BRK option.

Signed-off-by: Horst Schirmeier <horst@schirmeier.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>