[PATCH] page flags: add commentry regarding field reservation

Add some documentation regarding the utilisation of the flags field in
struct page.  This field is overloaded for per page bits and to hold node,
zone and SPARSEMEM information.  Make it clear which areas are used for
what and how many bits are in each area.

Signed-off-by: Andy Whitcroft <apw@shadowen.org>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>