kfence: Remove mention of PG_slab

Improve the documentation slightly, assuming I understood it correctly.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Acked-by: Harry Yoo <harry.yoo@oracle.com>
Link: https://patch.msgid.link/20250611155916.2579160-10-willy@infradead.org


Signed-off-by: Vlastimil Babka <vbabka@suse.cz>