Refine rbtree comment for clarity and accuracy

The comment now explains that each node requires at least one byte of
storage and contrasts the theoretical maximum node count with the
practical depth bound of 2 * log₂(n).

The rationale for using a fixed-size array to track the tree path during
operations is clearly described.

Author: jserv

src/map.c

@@ -30,12 +30,15 @@ struct map_internal {
     map_cmp_t (*comparator)(const void *, const void *);
 };
 
-/* Each node in the red-black tree consumes at least 1 byte of space (for the
- * linkage if nothing else), so there are a maximum of sizeof(void *) << 3
- * red-black tree nodes in any process (and thus, at most sizeof(void *) << 3
- * nodes in any red-black tree). The choice of algorithm bounds the depth of
- * a tree to twice the binary logarithm (base 2) of the number of elements in
- * the tree; the following bound applies.
+/* Each red–black tree node requires at least one byte of storage (for its
+ * linkage). A one-byte object could support up to 2^{sizeof(void *) * 8} nodes
+ * in an address space. However, the red–black tree algorithm guarantees that
+ * the tree depth is bounded by 2 * log₂(n), where n is the number of nodes.
+ *
+ * For operations such as insertion and deletion, a fixed-size array is used to
+ * track the path through the tree. RB_MAX_DEPTH is conservatively defined as 16
+ * times the size of a pointer to ensure that the array is large enough for any
+ * realistic tree, regardless of the theoretical maximum node count.
  */
 #define RB_MAX_DEPTH (sizeof(void *) << 4)