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/oid.h b/include/mbedtls/oid.h
index e4c697b..1c39186 100644
--- a/include/mbedtls/oid.h
+++ b/include/mbedtls/oid.h
@@ -41,8 +41,10 @@
 #include "mbedtls/md.h"
 #endif
 
-#define MBEDTLS_ERR_OID_NOT_FOUND                         -0x002E  /**< OID is not found. */
-#define MBEDTLS_ERR_OID_BUF_TOO_SMALL                     -0x000B  /**< output buffer is too small */
+/** OID is not found. */
+#define MBEDTLS_ERR_OID_NOT_FOUND                         -0x002E
+/** output buffer is too small */
+#define MBEDTLS_ERR_OID_BUF_TOO_SMALL                     -0x000B
 
 /* This is for the benefit of X.509, but defined here in order to avoid
  * having a "backwards" include of x.509.h here */