memblock: Document memblock_is_region_{memory,reserved}()

memblock: Document memblock_is_region_{memory,reserved}()

At first glance one would think that memblock_is_region_memory()
and memblock_is_region_reserved() would be implemented in the
same way. Unfortunately they aren't and the former returns
whether the region specified is a subset of a memory bank while
the latter returns whether the region specified intersects with
reserved memory.

Document the two functions so that users aren't tempted to
make the implementation the same between them and to clarify the
purpose of the functions.

Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>
Cc: Tejun Heo <tj@kernel.org>
Link: http://lkml.kernel.org/r/1337845521-32755-1-git-send-email-sboyd@codeaurora.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>