arm64: Update 'unwinder howto'

Implementing a new unwinder is a bit more involved than writing
a couple of helpers, so let's not lure the reader into a false
sense of comfort. Instead, let's point out what they should
call into, and what sort of parameter they need to provide.

Signed-off-by: Marc Zyngier <maz@kernel.org>
Reviewed-by: Kalesh Singh <kaleshsingh@google.com>
Tested-by: Kalesh Singh <kaleshsingh@google.com>
Reviewed-by: Oliver Upton <oliver.upton@linux.dev>
Link: https://lore.kernel.org/r/20220727142906.1856759-7-maz@kernel.org

arch/arm64/include/asm/stacktrace/common.h

@@ -5,17 +5,11 @@
  * To implement a new arm64 stack unwinder:
  *     1) Include this header
  *
- *     2) Provide implementations for the following functions:
- *          on_overflow_stack():   Returns true if SP is on the overflow
- *                                 stack.
- *          on_accessible_stack(): Returns true is SP is on any accessible
- *                                 stack.
- *          unwind_next():         Performs validation checks on the frame
- *                                 pointer, and transitions unwind_state
- *                                 to the next frame.
+ *     2) Call into unwind_next_common() from your top level unwind
+ *        function, passing it the validation and translation callbacks
+ *        (though the later can be NULL if no translation is required).
  *
- *         See: arch/arm64/include/asm/stacktrace.h for reference
- *              implementations.
+ * See: arch/arm64/kernel/stacktrace.c for the reference implementation.
  *
  * Copyright (C) 2012 ARM Ltd.
  */