lib min_heap: add brief introduction to Min Heap API

A short description of the Min Heap API is added to the min_heap.h,
explaining its purpose for managing min-heaps and emphasizing the use of
macro wrappers instead of direct function calls.  For more details, users
are directed to the documentation at Documentation/core-api/min_heap.rst.

Link: https://lkml.kernel.org/r/20241129181222.646855-4-visitorckw@gmail.com


Signed-off-by: Kuan-Wei Chiu <visitorckw@gmail.com>
Cc: Ching-Chun (Jim) Huang <jserv@ccns.ncku.edu.tw>
Cc: Geert Uytterhoeven <geert@linux-m68k.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>