[$] Improving memory-management documentation

Like much of the kernel, the memory-management subsystem is under-documented,
and much of the documentation that does exist is less than fully current.
At the 2022 Linux
Storage, Filesystem, Memory-management and BPF Summit (LSFMM), Mike
Rapoport ran a session on memory-management documentation and what can be
done to improve it. The result was a reinvigorated interest in
documentation, but only time will tell what actual improvements will come
from that interest.