mm: document highmem_is_dirtyable sysctl

It seems that there are still people using 32b kernels which a lot of
memory and the IO tend to suck a lot for them by default.  Mostly
because writers are throttled too when the lowmem is used.  We have
highmem_is_dirtyable to work around that issue but it seems we never
bothered to document it.  Let's do it now, finally.

Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Michal Hocko <[email protected]>
Acked-by: Johannes Weiner <[email protected]>
Cc: Alkis Georgopoulos <[email protected]>
Cc: Mel Gorman <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

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