Note out-of-date obmalloc comments (#148149)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

Author: tim-one

Include/internal/pycore_obmalloc.h

@@ -14,6 +14,12 @@ typedef unsigned int pymem_uint;  /* assuming >= 16 bits */
 #undef  uint
 #define uint pymem_uint
 
+/* NOTE: the following overviews were in the initial checkin, in 1998. In
+ * 2026, they're still helpful, but some details have changed. For example,
+ * we now use 32 size classes 16 bytes apart, and an arena is generally at
+ * least 1MB. Use sys._debugmallocstats() to see exact current details for
+ * the specific version of CPython used.
+ */
 
 /* An object allocator for Python.