Documentation: update the CONFIG_DEBUG_PAGEALLOC description

The CONFIG_DEBUG_PAGEALLOC option now is located under "Kernel
hacking" / "Memory Debugging" / "Debug page memory allocations".
so we should update the description in kmemcheck.txt.

Signed-off-by: Wang Long <long.wanglong@huawei.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>