gencgc: Commentary fix for struct page, field region_start_offset.

gencgc: Commentary fix for struct page, field region_start_offset.

  * Simply describing region_start_offset as being related to an
allocation region which contains a page is disingenuous at best,
and misleading at worst.  Its relation with an alloc_region is due
to its initialization strategy, and has nothing to do with what
the value is for.

  * Say it like it is, it's an offset to a known object boundary,
from where we can start a call to gc_search_space() or scavenge().
That's what it's for, not for keeping track of alloc_regions.