Revert "kernel/vm: Remove an obsolete comment."

This reverts commit 1db0961121.

It turns out the comment is not obsolete; what it refers to isn't
PAE systems but true 32-bit ones. I'm not sure we should use
64-bit cache offsets even there, but that's a decision for another
time.

headers/private/kernel/vm/vm_types.h

@@ -123,6 +123,8 @@ private:
 public:
 	page_num_t				cache_offset;
 								// in page size units
+								// TODO: Only 32 bit on 32 bit platforms!
+								// Introduce a new 64 bit type page_off_t!
 
 	SplayTreeLink<vm_page>	cache_link;
 	vm_page*				cache_next;