doc: Add slab internal kernel-doc

We don't have much real internal documentation to extract yet, but
let's make sure that what we do have is available.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Acked-by: Harry Yoo <harry.yoo@oracle.com>
Link: https://patch.msgid.link/20250611155916.2579160-7-willy@infradead.org


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