init: Document that ReapOneProcess() does not modify 'pid'

It is essential for correct operation of ReapAnyOutstandingChildren()
that ReapOneProcess() does not modify 'pid'. Make it easier to verify
for humans that ReapOneProcess() does not modify the 'pid' local
variable. Document the value of siginfo.si_signo with DCHECK_EQ()
statements.

Bug: 213617178
Change-Id: I14baaa7adbe5416b7f777f32fb76896f2ae27b4e
Signed-off-by: Bart Van Assche <bvanassche@google.com>