Note out-of-date obmalloc comments (#148149)

Vladimir's original overviews, from 1998, are still good, but going
on 30 years later details have changed. Note that, but rather try
to keep up with moving targets in a different file, point to
sys._debugmallocstats() as the sure way to discover precise current
details.

No code changes, just added a block comment.

Co-authored-by: Bartosz Sławecki <bartosz@ilikepython.com>