atomic.h: Reword confusing comment for qatomic_cmpxchg

The qatomic_cmpxchg() and qatomic_cmpxchg__nocheck() macros have
a comment that reads:
 Returns the eventual value, failed or not

This is somewhere between cryptic and wrong, since the value actually
returned is the value that was in memory before the cmpxchg.  Reword
to match how we describe these macros in atomics.rst.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Zhao Liu <zhao1.liu@intel.com>
Message-id: 20240223182035.1048541-1-peter.maydell@linaro.org

include/qemu/atomic.h

@@ -202,7 +202,7 @@
     qatomic_xchg__nocheck(ptr, i);                          \
 })
 
-/* Returns the eventual value, failed or not */
+/* Returns the old value of '*ptr' (whether the cmpxchg failed or not) */
 #define qatomic_cmpxchg__nocheck(ptr, old, new)    ({                   \
     typeof_strip_qual(*ptr) _old = (old);                               \
     (void)__atomic_compare_exchange_n(ptr, &_old, new, false,           \