lib/list_sort: clarify comparison function requirements in list_sort()

Add a detailed explanation in the list_sort() kernel doc comment
specifying that the comparison function must satisfy antisymmetry and
transitivity.  These properties are essential for the sorting algorithm to
produce correct results.

Issues have arisen in the past [1][2][3][4] where comparison functions
violated the transitivity property, causing sorting algorithms to fail to
correctly order elements.  While these requirements may seem
straightforward, they are commonly misunderstood or overlooked, leading to
bugs.  Highlighting these properties in the documentation will help
prevent such mistakes in the future.

Link: https://lore.kernel.org/lkml/20240701205639.117194-1-visitorckw@gmail.com [1]
Link: https://lore.kernel.org/lkml/20241203202228.1274403-1-visitorckw@gmail.com [2]
Link: https://lore.kernel.org/lkml/20241209134226.1939163-1-visitorckw@gmail.com [3]
Link: https://lore.kernel.org/lkml/20241209145728.1975311-1-visitorckw@gmail.com [4]
Link: https://lkml.kernel.org/r/20250106170104.3137845-3-visitorckw@gmail.com
Signed-off-by: Kuan-Wei Chiu <visitorckw@gmail.com>
Cc: Ching-Chun (Jim) Huang <jserv@ccns.ncku.edu.tw>
Cc: <chuang@cs.nycu.edu.tw>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Author: visitorckw

lib/list_sort.c

@@ -108,6 +108,13 @@ static void merge_final(void *priv, list_cmp_func_t cmp, struct list_head *head,
  * and list_sort is a stable sort, so it is not necessary to distinguish
  * the @a < @b and @a == @b cases.
  *
+ * The comparison function must adhere to specific mathematical properties
+ * to ensure correct and stable sorting:
+ * - Antisymmetry: cmp(@a, @b) must return the opposite sign of
+ * cmp(@b, @a).
+ * - Transitivity: if cmp(@a, @b) <= 0 and cmp(@b, @c) <= 0, then
+ * cmp(@a, @c) <= 0.
+ *
  * This is compatible with two styles of @cmp function:
  * - The traditional style which returns <0 / =0 / >0, or
  * - Returning a boolean 0/1.