x86 boot: longer comment explaining sanitize_e820_map routine

Elaborate on the comment for sanitize_e820_map(), epxlaining more what
it does, what it inputs, and what it returns.  Rearrange the placement of
this comment to fit kernel conventions, before the routine's code rather
than buried inside it.

Signed-off-by: Paul Jackson <pj@sgi.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>