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/base64.h b/include/mbedtls/base64.h
index 7e73a8b..cf4149e 100644
--- a/include/mbedtls/base64.h
+++ b/include/mbedtls/base64.h
@@ -30,8 +30,10 @@
 
 #include <stddef.h>
 
-#define MBEDTLS_ERR_BASE64_BUFFER_TOO_SMALL               -0x002A  /**< Output buffer too small. */
-#define MBEDTLS_ERR_BASE64_INVALID_CHARACTER              -0x002C  /**< Invalid character in input. */
+/** Output buffer too small. */
+#define MBEDTLS_ERR_BASE64_BUFFER_TOO_SMALL               -0x002A
+/** Invalid character in input. */
+#define MBEDTLS_ERR_BASE64_INVALID_CHARACTER              -0x002C
 
 #ifdef __cplusplus
 extern "C" {