x86/mm: further clarify switch_mm_irqs_off() documentation

Commit accf6b23d1e5a ("x86/mm: clarify "prev" usage in
switch_mm_irqs_off()") attempted to clarify x86's usage of the arguments
passed by generic code, specifically the "prev" argument the is unused by
x86.  However, it could have done a better job with the comment above
switch_mm_irqs_off().  Rewrite this comment according to Dave Hansen's
suggestion.

Link: https://lkml.kernel.org/r/20240222190911.1903054-1-yosryahmed@google.com
Fixes: 3cfd6625 ("x86/mm: clarify "prev" usage in switch_mm_irqs_off()")
Signed-off-by: Yosry Ahmed <yosryahmed@google.com>
Suggested-by: Dave Hansen <dave.hansen@intel.com>
Acked-by: Dave Hansen <dave.hansen@intel.com>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Borislav Petkov (AMD) <bp@alien8.de>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

arch/x86/mm/tlb.c

@@ -493,10 +493,10 @@ static inline void cr4_update_pce_mm(struct mm_struct *mm) { }
 #endif
 
 /*
- * The "prev" argument passed by the caller does not always match CR3. For
- * example, the scheduler passes in active_mm when switching from lazy TLB mode
- * to normal mode, but switch_mm_irqs_off() can be called from x86 code without
- * updating active_mm. Use cpu_tlbstate.loaded_mm instead.
+ * This optimizes when not actually switching mm's.  Some architectures use the
+ * 'unused' argument for this optimization, but x86 must use
+ * 'cpu_tlbstate.loaded_mm' instead because it does not always keep
+ * 'current->active_mm' up to date.
  */
 void switch_mm_irqs_off(struct mm_struct *unused, struct mm_struct *next,
 			struct task_struct *tsk)