x86: add comments to describe the new api's in cacheflush.h

The new cacheflush.h API's didn't have any comments describing
how they're to be used yet and the conventions around these functions.
This patch adds comments to this effect; in order for that to be
a logical series, some prototypes had to move around.

Signed-off-by: Arjan van de Ven <arjan@linux.intel.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>