gh-74573: document that ndbm can silently corrupt databases on macOS (#113354)

* gh-74573: document that ndbm can silently corrupt databases on macOS

The system ndbm implementation on macOS has an undocumented limitation
on the size of values and can silently corrupt database files when those
are exceeded.

Co-authored-by: Erlend E. Aasland <erlend.aasland@protonmail.com>