brk: document randomize_va_space and CONFIG_COMPAT_BRK (was Re:

Document randomize_va_space and CONFIG_COMPAT_BRK.

Signed-off-by: Jiri Kosina <jkosina@suse.cz>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>