Note out-of-date obmalloc comments (How To Make Instagram Posts Less Boring)

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>

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@@ -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.