gencgc: Fix commentary for page table allocation field.

gencgc: Fix commentary for page table allocation field.

  * The commentary for the page table allocation field was
misleading, presumably not updated when the definitions for the
constants used for its actual contents were last changed, and cost
me a bit of surprise and time spent trying to figure out why core
file saving and loading worked at all.

  * Updated the commentary on the allocation field to match
current reality, and added cross-references between the field
itself and the definitions for its contents, so that a future
desync between commentary and reality is less likely.