Move MBEDTLS_ERR_xxx Doxygen comments before the definition

Now that descriptions of error codes no longer have to be on the same line
for the sake of generate_errors.pl, move them to their own line before the
definition. This aligns them with what we do for other definitions, and
means that we no longer need to have very long lines containing both the C
definition and the comment.

```
perl -i -pe 's~^(#define +MBEDTLS_ERR_\w+ +-\w+) */\*[*!]<(.*)\*/~/**$2*/\n$1~' include/mbedtls/*.h
```

This commit does not change the output of generate_errors.pl.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
diff --git a/include/mbedtls/threading.h b/include/mbedtls/threading.h
index 05e27c5..d147c73 100644
--- a/include/mbedtls/threading.h
+++ b/include/mbedtls/threading.h
@@ -36,10 +36,13 @@
 
 /* MBEDTLS_ERR_THREADING_FEATURE_UNAVAILABLE is deprecated and should not be
  * used. */
-#define MBEDTLS_ERR_THREADING_FEATURE_UNAVAILABLE         -0x001A  /**< The selected feature is not available. */
+/** The selected feature is not available. */
+#define MBEDTLS_ERR_THREADING_FEATURE_UNAVAILABLE         -0x001A
 
-#define MBEDTLS_ERR_THREADING_BAD_INPUT_DATA              -0x001C  /**< Bad input parameters to function. */
-#define MBEDTLS_ERR_THREADING_MUTEX_ERROR                 -0x001E  /**< Locking / unlocking / free failed with error code. */
+/** Bad input parameters to function. */
+#define MBEDTLS_ERR_THREADING_BAD_INPUT_DATA              -0x001C
+/** Locking / unlocking / free failed with error code. */
+#define MBEDTLS_ERR_THREADING_MUTEX_ERROR                 -0x001E
 
 #if defined(MBEDTLS_THREADING_PTHREAD)
 #include <pthread.h>