slabinfo: add comment for cache_size calculation

One of our physical machine shows that the "CACHE SIZE" column of slabtop output is extremely high, three times of the products of objs nums and objs size. After some analysis, we found that the order of slab, which decides "pages per slab", will shrink when memory pressure is high and normal order allocation failed. So we think it might help to add these comments to the man help.

Minor fix: add the "memory." back, which is lost after "aa461df0: docs: Minor manpage fixes"

Signed-off-by: Zhao Mengmeng zhaomengmeng@kylinos.cn