Add an explaining comment for _Py_HAMT_MAX_TREE_DEPTH

Author: 1st1

immutables/_map.h

@@ -4,6 +4,17 @@
 #include <stdint.h>
 #include "Python.h"
 
+/*
+HAMT tree is shaped by hashes of keys. Every group of 5 bits of a hash denotes
+the exact position of the key in one level of the tree. Since we're using
+32 bit hashes, we can have at most 7 such levels. Although if there are
+two distinct keys with equal hashes, they will have to occupy the same
+cell in the 7th level of the tree -- so we'd put them in a "collision" node.
+Which brings the total possible tree depth to 8. Read more about the actual
+layout of the HAMT tree in `_map.c`.
+
+This constant is used to define a datastucture for storing iteration state.
+*/
 #define _Py_HAMT_MAX_TREE_DEPTH 8