arch/posix: move fuzz entry doc to the right place

It should document the entry point function, not the DMA-like variable.

Signed-off-by: Michael Zimmermann <sigmaepsilon92@gmail.com>

boards/posix/native_posix/main.c

@@ -120,16 +120,16 @@ int main(int argc, char *argv[])
 
 #else /* CONFIG_ARCH_POSIX_LIBFUZZER */
 
+const uint8_t *posix_fuzz_buf;
+size_t posix_fuzz_sz;
+
 /**
  * Entry point for fuzzing (when enabled). Works by placing the data
  * into two known symbols, triggering an app-visible interrupt, and
  * then letting the OS run for a fixed amount of time (intended to be
  * "long enough" to handle the event and reach a quiescent state
  * again)
  */
-const uint8_t *posix_fuzz_buf;
-size_t posix_fuzz_sz;
-
 int LLVMFuzzerTestOneInput(const uint8_t *data, size_t sz)
 {
  static bool posix_initialized;