Document why the LOS space is not scanned during Sticky-Bit CC collections.

Add a comment explaining why the large-object (LOS) space is not
scanned during sticky-bit (young-generation) CC collections at the
beginning of ConcurrentCopying::MarkingPhase, whereas non-immune
continuous spaces (region space and non-moving space) are.

Test: n/a
Bug: 67628039
Bug: 12687968
Change-Id: If560e8e66a25b50052d25e7e791a53b09508c024