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/xtea.h b/include/mbedtls/xtea.h
index 473dd4b..4bdc711 100644
--- a/include/mbedtls/xtea.h
+++ b/include/mbedtls/xtea.h
@@ -34,10 +34,12 @@
 #define MBEDTLS_XTEA_ENCRYPT     1
 #define MBEDTLS_XTEA_DECRYPT     0
 
-#define MBEDTLS_ERR_XTEA_INVALID_INPUT_LENGTH             -0x0028  /**< The data input has an invalid length. */
+/** The data input has an invalid length. */
+#define MBEDTLS_ERR_XTEA_INVALID_INPUT_LENGTH             -0x0028
 
 /* MBEDTLS_ERR_XTEA_HW_ACCEL_FAILED is deprecated and should not be used. */
-#define MBEDTLS_ERR_XTEA_HW_ACCEL_FAILED                  -0x0029  /**< XTEA hardware accelerator failed. */
+/** XTEA hardware accelerator failed. */
+#define MBEDTLS_ERR_XTEA_HW_ACCEL_FAILED                  -0x0029
 
 #ifdef __cplusplus
 extern "C" {